Don't assume that the cost of living here is "way cheaper" than London. It's not, and it's getting more expensive all the time.

It would help if you mention whether you're single or married.

You will find many recent posts/threads about accommodation and living expenses in Qatar.

I would avoid giving them any specific figures. Consider the whole package on offer; accommodation, transportation, medical and education allowances 1 and 4 being the most expensive. Then decide how much you want to have in-pocket net.

Generally speaking I think you should stick to your 35k as a minimum base which will work out to about QAR 20,000 monthly salary but like I said before consider the whole package.