I dont think they would need to be here for 12 months to get a P60.
Apart from fact that if moved in May 2006 they would get a P60 in December 2006. Most banks will look at more than just the P60.
It will depend on the type of job, ie which sector, Part time vs full time, contract or permanent as well as the salary. This will be required for both parties.
They will look at disposable income, existing expenses, loans etc.
We moved from Uk to Ireland and got a mortgage in less than 12 months, OH was here 8 months and I was here only 3 months when approved.
Main thing is open and start using a bank account as soon as they return