Assumptions:
7.5 applicants per place (Imperial was 7.2 for 2010 entry).
One in three offers are firmed
Some (say 20%) of people who firm miss offers and no people enter via insurance, every one applies without deferred entry.
So, make an offer to 50% of applicants, 16.6% of the total applicants firm it, 13.3% of total applicants meet it - ie 7.5 applicants per place.
Rationale:
1/3 of offers firmed: You apply to similar universities with very little to differ between them. If you get an offer from one university it is very likely that another 2 of the five would give you an offer. Essentially it's a random decision whether you might choose Bristol over Durham over UCL.
Missing offers: I think it's very generous to say 80% of people who firm meet their offer.
Applicants per place: I've taken a rough estimate from
Imperial's admissions stats. Yes,
Bristol (offers A*(M)AA or A(M)A(FM)A) has something nearer 13 applicants per place, but
Durham (A*A(M/FM)A) has 8 applicants per place so I think it's a fair assumption.
The above assumptions are horrendously flawed but I just did some digging and brought up some interesting results:
Durham are very open with admissions figures so according to
this site so you can actually have a look at a department application numbers, the number of offers, the number of replies (be it firm or insurance) and the number of AS12's (ie incoming students).
For the 2009-10 admissions cycle, Durham had:
1145 applicants, of which they made
507 offers (be it conditional or unconditional), of which
273 firmed or insured, of which
143 entered Durham either in 2010 or deferred to 2011.
So Durham (with a comparatively easier A*AA offer) made offers to 44.3% of applicants, of which 53.8% of them firmed or insured and of them 52.4% eventually got into Durham - giving 8 applicants per place. That's still a high offer percentage, no?