Most vets will allow between 15-18mths between boosters so you should be fine. There is also a lot of controversy about whether an adult cat even needs a full course if its overdue on its boosters. So if only a few weeks over the date, you should be absolutely fine.
As said above there is a time limit on when they can have boosters before it goes back to having to have the full set again. Best bet is to ask vets maybe ask if they can do a part payment deal or something.
The problem is that if you miss the booster date by more than a couple of weeks then you won't be able to have the booster at all. Instead you will have to start again with two vaccines three weeks apart, which will cost twice as much. I would check with the vet how long the gap could be before you have to start all over again, but it's certainly not long. If you're in London Celia Hammonds only charges £10 per vaccination.
He is so disinterested in outside and terrified of the other cats! We'll be able to do it in a couple of weeks - just typically had loads of outgoings this week <am capable of providing for a cat - honest!!>
Our lovely rescue cat is due his boosters this week - we got him in January, and of course he is insured, but we really can't afford to have them done this week - car tax, bills, living Will he be ok if we leave it for another month? he rarely goes outside yet.