Well done on your savings, that's an amazing amount to accrue - is that on top of the £1000 a month overpayment? Very impressive!
As to what to do with it, I think with that amount of money to spend you'd be justified in paying for some proper professional independent financial advice. The best thing to do depends on a number of personal circumstances - you don't have to answer these questions here but an IFA will ask/take into account - the ages of you and your partner and your pension situation, whether you have the option to make additional voluntary contributions to your pension schemes and the value of these, whether you have already maxed out your ISA allowances, the current interest rate on your mortgage and how long you have left on the fixed term deal (and bear in mind that the possibility of recession over the next few years may mean it's harder to get a good mortgage deal if you are trying to remortgage in the next few years), whether you have any children and their ages, whether you want to earmark or start a fund for their benefit, and finally your risk/reward appetite, ie are you prepared to risk losing some of the money to earn more, or do you want to take the safest possible route.
All other things being equal, if it was me personally I would probably leave a good chunk in an easily accessible account as a rainy day fund, the equivalent probably of 6 months income for you and your partner (to cover eventualities such as job loss, severe illness, urgent house repairs etc), would pay a substantial chunk off the mortgage, and then invest the remainder in a mix of ISAs, pension fund and managed investment fund that matches your level of risk appetite - personally (well pre-Covid anyway) my investments have always outperformed the money I would save by putting the equivalent amount into paying off the mortgage early, but I have a medium-high risk appetite so this wouldn't be the case for everyone. So there might well be a good argument depending on your mortgage rate to simply focus for the time being on paying off your mortgage and then starting to build other investments with the money you save on the mortgage further down the line.
Perhaps also if the £100k mark is a milestone, now is the time to 'treat' yourself and your DH/DP a little? I'm not talking about frittering away the lot on crap, and I too am a real saver rather than spender so struggle with this, but allowing yourself to live a little on the odd occasion can be what keeps you on the saving path for the next 10 years, so perhaps book a holiday or do that home improvement work you've always wanted or buy yourself that 'luxury' item you've been craving? You do deserve it after all those years frugality!