Joe24 said:
I weigh 9 and a half stone(sometimes a few pounds over that) so how many sets?
I used to bench 35-40kgs when i was in the gym and do sets of 12, 10, 8.
A large proportion of my weight is in my legs aswell, with only a small amount in my upper body.
I dont float either

I went swimming with my GF in the holidays, and when i lent back to try and float, i just ended up sinking, well my legs sinking
theoretically, the force for a press up is half your body weight, as one half is supported by your feet, so if you're 60kg, it's like bench pressing 30kg.
However, that approximation assumes that:
* you are of even linear density, i.e. your top half weighs the same as your legs
* the amount you rise is negligible compared to the distance from hands to feet (i.e. that you are infinitely tall)
* you are horizontal.
if you start off an an incline (or technically, are at an incline at any point during the pressup) then trigonometry comes into it - as MacB points out with 'inlined' press ups.
These assumptions are probably valid for the purposes of the approximation, but you can make it easier by having your head higher than your feet, and make it harder by having your head lower than your feet.
How much easier for a given angle would require a trigonometric calculation which I could do but is too complex for me to do off the top of my head. If you really really want me to work it out let me know and i will
But possibly just as good will be to go incrementally - i.e.if it's too easy make it a bit harder, by doing more of them in a set or putting books/blocks under your feet.
The important thing to keep it even is keep your body straight, i.e. don't bend at the waist.