Congratulations.
It is of course very difficult to comment without a fuller picture of your situation, your ages, incomes, tax situation, mortgage interest rate, future plans etc.
However your choice comes down to some combination of paying off the mortgage and investing. You sound quite risk averse and uncertain of investing. Paying off the mortgage is probably going to be the best risk free investment you can make, although I've no idea what interest rate you are paying.
You need to learn more about investing, I recommend reading Smarter Investing by Tim Hale and the Monevator blog. Avoid the noise generated by the mainstream money media.
There is no harm in going to see an IFA, but make sure you have educated yourself before you go so you can decide if they can offer you anything. Only pay an IFA if you have some particularly complex situation, or feel that you are unable to cope with opening an online broker account and purchasing Vanguard Lifestrategy funds for yourself, or enjoy paying someone else thousands of pounds a year.
You want to make as much use of ISA's and pensions as you can to shelter the money from tax.
You might want to keep some money in cash for the potential £80k IHT liability, or alternatively be prepared to remortgage the house if necessary.
Other than that, make sure you have one year of your living expenses in cash.
If you want to move house now, then move house now. If you know you want to move house in 3 years time and need X amount of money at that point you might put X amount of money in a 3 year term savings bond/account.
If you already have 2 houses, I suggest that you have enough property already and need to diversify.
No need to rush into doing anything while you are still uncertain. Take your time to learn more and consider the options.