No set figure, but probably around £900 maximum, preferable lower than though.
The pawn shop option, could be a good idea. Thanks for the suggestion Paul.
The general rule I have used when advising on this over the years is purchase the fastest processor, most RAM and largest hard disk within the set budget. Essentially, you have to buy a new laptop/PC. If you want to you could keep adding options like a touch sensitive screen, dedicated graphics by NVidia, aluminium casing, super thin/lightweight etc. and so spend many thousands of pounds for one.
For your budget look for an i7 Intel processor or equivalent, 8 gigabytes of RAM and at least 0.5 TB hard disk. And even though you get the operating system with it don't forget that you will need software to run on it - so factor in the cost of Microsoft Office, Scrivener etc. as well. Your budget Geoff will definitely buy you a laptop with those specifications so next we have consider your own preferences.
Some people have a long standing preference for a particular maker - mine always leans towards Toshiba because I have bought a lot of their machines in the past and their product support/drivers are excellent. Others prefer Dell, Sony, Acer, Lenovo, etc. etc. That is apart from the look of the thing. So the best thing to do is have a look around on the PC World website and others and then post the link to the product you like the look of in this thread and I can give you feedback on it.
One tip - laptop cases are often eye wateringly expensive. I bought mine in Wilkinson's for a tenner and I can't tell the difference between that one and cases that others have bought for five times the price. Unless you want a brand or designer name consider doing the same.