I was going to post this as a response in the comments to this one by Roger Helmer MEP, but as it's long and technical I'll make it a new post in itself.
It is not strictly true to say that posts in the European Parliament are allocated on a per head basis - there is a bias towards larger groupings - but also if an alternative group is to be formed then most of the other parties involved would not be automatically entitled to positions such as chairs and vice chairs and we Conservatives would probably have to give them some of our allocation just to get & keep them on-board.
The allocation formula is D'Hondt, which has a bias towards larger groups and coalitions - see this conference presentation for an example of the number crunching. One practical outcome is that the European People's Party-European Democrats group currently has one more committee chair than a strict "per head" basis.
To my awareness the best analysis of not only the likely numbers involved but of the entire situation is P. Lynch & R. Whitaker (2008) "A Loveless Marriage: The Conservatives and the European People's Party." Parliamentary Affairs 61(1) (January 2008): 31-51. (Those with access can see the article via the online archive.) In the article (amongst other matters) Lynch & Whitaker modelled two hypothetical groups in the European Parliament in 2007 to demonstrate the numbers and allocation. (Whilst the 2009 elections will bring changes, they do not anticipate these to be so great as to have a significant impact on the numbers involved.) The two groups include a "smaller" one, based on both a limited number of recruits and six Conservative MEPs staying in the EPP-ED (although Ulster Unionist Jim Nicholson would be in), and a "larger" one with all the Conservatives on-board plus further countries. The impact on posts is clear.
For the record the smaller group consists as follows:
1. 22 Conservatives & Unionists (with six staying in the EPP-ED) - UK
2. 9 Civic Democrats (ODS) - Czech Republic
3. 7 Law and Justice (PiS) - Poland
4. 4 For Fatherland and Freedom/LNNK (LNNK) - Latvia
5. 1 Pensioners' Party - Italy
6. 1 Independent - Kathy Sinnott - Ireland
Total: 44 MEPs
The larger group consists of all the above, the remaining six Conservatives and the following:
7. 4 Centre Party - Finland
8. 3 Movement for France (MPF) - France
9. 2 Social Democratic Centre–People's Party (CDS-PP) - Portugal
10. 2 United Democratic Forces (UDF) - Bulgaria (and this was based on the January to May appointed interim delegation before the UDF failed to win any seats in the subsequent election)
Total: 61 MEPs
Now others can comment on the ease of obtaining each of these nine, or for that matter whether they will have seats after the elections (e.g. the UDF didn't get anyone elected in the 2007 elections and this led to a change of leadership that makes them more unlikely to join up if they get the chance). But these hypothetical groups show well who would get the gains and losses.
The allocation of committee chairs and vice chairs becomes a factor here. The parties that would be in the smaller group currently have between them 3 chairs and 6 vice chairs. In the larger group it's 3 chairs and 10 vice chairs. But if the new groups were formed then the smaller group gets 1 chair and 4 vice chairs. The larger group gets 1 chair 6 vice chairs.
And assuming the use of the D'Hondt formula to divide up the allocations within the group then the losses overwhelmingly hit the other parties, not the Conservatives, again because of the D'Hondt bias towards larger parties. In the smaller group, the Conservatives will retain 1 chair and 3 vice chairs. In the larger group the Conservatives actually gain another vice chair entitlement. So from a Conservative point of view the raw numbers suggest there's nothing to lose (but we'll come to the question of which committees) and possibly a little bit to gain. But the changes for the other parties are as follows (smaller/larger):
ODS -1 chair/-1 chair
PiS -1 chair/- 1 chair + 1 vice chair
LNNK -1 vice chair/-1 vice chair
Pensioners same/same (nothing to start with)
Kathy Sinnott -1 vice chair/-1 vice chair
Centre Party NA/-3 vice chairs
MPF NA/same (nothing to start with)
CDS-PP NA/-1 vice chair
UDF NA/same (nothing to start with)
So what exactly is in it for these parties to move away from their current groups? And remember some of these groups are already quite Eurosceptic so it's not the same issue as between us and the EPP. The best I can see is that the Conservatives would have to hand over some of their entitlement, along with other posts in both the parliament and the group to other parties (e.g. giving the ODS the post of group leader).
And this doesn't even address the fact that as well as the drop in raw numbers, the choice of committees is allocated on the basis of group sizes. The larger group would get onky the 8th choice, the smaller group the 12th choice. These are not going to be the most influential committees.
Similarly the rapporteurs, who "prepare reports on bills that set out the EP's position and propose amendments", are allocated in a similar way so once again it is the large groupings who will get proportionally more rapporteurships and, crucially, the more important bills. (And once again the Conservatives might find they have to give over some entitlement to others in the new group.)
Now all this talk about posts in the European Parliament does not in itself matter one iota to voters. What does matter is output. It is the holding of posts that enhances a party's ability to influence the output of the European Parliament. It is here that MEPs can best stand up for the interests of the public and get the best results.