Usual caveat: I am not a qualified financial adviser... So, in no particular order:
Where do you want to start? Do you want to commit financially as well as moving in? If yes, maybe pay 55k off her mortgage?
Clear ALL outstanding debts - especially store or credit cards - then cut up the cards (keep one for emergencies - perferably one with a 0% deal on purchases).
Use your ISA allowances for this tax year (ends 5 April): 7k in a maxi ISA or 3K in a cash mini ISA and 4k in a stocks & shares ISA; you can invest a bit more in the next tax year (I think it's 7.2k).
Aim to have 3-6 months salary/outgoings reasonably easily available in case of redundancy/illness etc.
If you want to invest, where you do so depends on your appetite for risk. You could put up to 30k in Premium Bonds - no interest but should, on current odds, win about 15 prizes p.a. and one could be �1m.
High interest notice accounts are reasonably safe, if dull - (NB - no more than �35k with any one provider as that's the maximum protected amount).
Long term investment options lead to shares; FTSE tracker funds are about as safe as share investment gets (NO share is ever completely safe); investing in individual stocks is riskier but if you hit the right one could be massively beneficial.
And if your attitude to risk is extremely high, fly to Las Vegas... or put it on Laughing Boy in the 3.30 at Kempton....
Finally, if you are still stuck, just let me have it.
The most sensible option is to take a paid-for appointment with a fee based Independent Financial Adviser who can discuss all of these things and more - including retirement planning, shool fees/university fees planning for children (if you have any/are planning on having any) and accident, sickness and unemplouyment insurance as well as critical illness cover etc.