When should payment be accounted for, on receipt or on provision of service?

Hi everyone,

I am self employed and I received a payment from a company at the end of last year (approx £37.5k) for a job that has not yet started (we are still finalising the spec) but they wanted to get it within that corporate accounting period. I was expecting we would have started by now but it won't be until sometime in March so it will span across this and next tax years. I just wanted to confirm which year i should account for it in, I'm pretty sure i'm right in saying that it is based on when the service is provided but i just wanted to make 100% sure? I would presume if 1/3 of the work is done in 11/12 and 2/3 in 12/13 (for example) then i should account for it in that proportion across the two years irrelevant of when the payment is made?

Thanks so much for your help,

Dave