How Big is your Salary? How much do you earn?
What sort of interest are you getting on your joint account?
Surely with £1k/month spare you are squirrelling some away into higher interest accounts (instant access for emergency), stock ISAs and Pensions?
If you don’t want to / can’t pay YOUR earnings direct from your employer into a sole account then set up a Standing Order from the joint account into your sole account that she does not have access to.
If she asks why? Say you have done it to get the better interest rate, or cash bonus for switching banks, or the higher interest rates on linked regular savings accounts.
The more bank accounts and credit cards you have the harder it will be for her to check, plus don’t give her access to login/passwords.
HBOS, Lloyds, Santander all pay interest on current account balances, plus you can get other perks.
Chase gives 1% cash back on spending, and 1.5% on easy access savings.
You can get more interest on regular savings:
FIrst Direct 3.5%
NatWest 3.25%
Nationwide 2.5%
Pay for all legit stuff on 0% credit cards, and stash the cash in any of the above to pay off when the 0% intro rate expires.
With £1k a month spare and £1k a month on daily spending you will soon have £20k across multiple saving accounts earning circa £300 -500/yr in interest/cash back to pay towards/hide the money spent on punts and gambling addictions.