Paddy - I posted earlier in this thread (28/11) - I'd suggest stay clear of multiples - I'm assuming you are in PAYE type employment, with reasonably regular wages (weekly/monthly).
I'm not asking you to answer these questions on this forum, but suggest you do you so in your own time ...
a) Net take home pay - say monthly
b) Fixed amounts that have to be paid from same - e.g. car loan, petrol - exclude rent if your plan is to live in the property you intend to purchase - include any other fixed amounts (what I'm getting at here is lifestyle regular spending)
c) Analyse your circumstances - married/single, dependants - set aside any fixed costs here (leave yourself/selves enough to live on)
d) Set aside a 'fixed' amount, include some level for contingencies.
When you have all this done - see how much you have left over - that's the amount you have for your mortgage/life cover - use this figure to calculate what mortgage amount it would buy you. Then see what that mortgage would cost you if rates rose by say 2% - adjust accordingly.
An interesting test (on the basis you don't plan to purchase something tomorrow morning), would be to set this amount aside for the next few months in to a savings account - it will prove to yourself how much you can really afford and can you manage it.
Apologies if the above sounds like a lecture - I don't mean it to be.
I hope it's of some help.
Regards,
BM