Sorry to hear about your parents, don't feel under pressure to make any quick decisions 
I would 100% seek some proper independent financial advice rather than going off what people on the internet tell you, it will be very dependent on you (and your DH/DPs) personal circumstances, your jobs/work, the number of DC and their ages (e.g. are they approaching the age where you'll be wanting to help them with university costs, first cars, house deposit, weddings etc), your housing situation and whether you want to sell/move any time soon, your attitude to investment and risk, long term growth vs immediate income and return, your pension situation and retirement plans, loads of stuff which you shouldn't go into detail about online as too identifying.
Paying at least some off the mortgage is probably a good idea, but again it depends on what interest rate you are paying compared to what you could expect to gain from investments, and what overpayment penalty you have on your current deal. You will also want to consider what you do with the money you then save on monthly repayments, spend it on improving your current lifestyle or save/invest for the future or a combination of both?
I think probably you want to allocate the majority of the money to long term investments (some combination of paying off the mortgage, maxing out pensions and ISAs and then a balanced investment portfolio for the remainder managed by a professional), but you should consider allowing yourself some more short term spending, e.g. as others have said, any home improvements, allowing yourself a bit more work flexibility, maybe even a holiday or car. I'm sure your parents would want to see you enjoy yourself a little as well as wisely investing the majority.