Ongoing problems with Ford Focus now out of warranty

I bought my Ford Focus in April 16 (it was around 18 months old) and within a few weeks started to experience problems with the tyres losing pressure. It has been back and forwards to the garage ever since with them replacing valves and messing around trying to seal the valves. In July 17 the fault was finally diagnosed as corroded alloys that needed to be replaced. One wheel was replaced at that time, even though the mechanic said that all 4 were 'going' (this isn't in writing though). The warranty expired in August. The wheel that was replaced is now fixed and doesn't lose pressure at all.

I am still having the same issues with the other 3 wheels, but because the car is now outside its 3 year warranty, the garage are telling me they won't replace the wheels. They have now made me a 'final offer' of 70% of the cost of 3 new wheels, leaving me with a bill for the balance of around £300.

I have told them that I am unhappy with this but am not getting anywhere. Do I have any rights under consumer protection laws? How do I proceed?

Thankyou