Assuming that a '1 bed flat in Zone 2/3' is a significant six figure sum, you're probably one of the few where it's worth talking to an IFA. Under around £100k or so, most people can just pay off (mortgage) debt, keep some in cash in various types (instant access, fixed, premium bonds etc) and fill up their S&S ISA allowance - you can deal with £40k of this by early April assuming you haven't already used this year's allowance.
For you, if you have a mortgage, then overpaying/paying off when any fix ends is likely to be a good low risk strategy. Likewise putting money you don't need before pension age into a SIPP, again within the allowances is likely to be worthwhile. Also, using up your S&S ISA allowance with Vanguard tracker funds as already mentioned. Obviously keep some cash for an emergency fund and any expected large purchases like new car, home improvements etc. Perhaps spend some of it on a treat like a big holiday to remember the relative by?
This financial flow chart will help you tick off possible actions
ukpersonal.finance/flowchart/
Also the Meaningful Money podcast. This is by an IFA who spends a lot of time telling people they don't need an IFA. He's also written a book and there's a lot of information on the website.
But if, after paying off your mortgage, using up your pension and ISA allowances, and keeping money for the short term, if you have money left, then you might want to talk to an IFA about what to do with the rest. Or you might want to just spend it, or give some to charity. Or start a business, or travel the world. It depends on your age and starting position.