I'd take the cash, the remap will only potentially make it less reliable and costly in the long run, when you bought the car to save money in the first place.
I'd take the £300 and take it as a lesson learnt.
£300 is still what, 3-4 tanks of fuel?
It was done at 60k and again when I bought it at 100 (part of the deal)
Also found through speaking to arval that it had a new clutch and flywheel at 90 and an EGR at 80
Now you know you should really inform your insurance company.
If the information is available to you on whats been done to it in the past then you can bet they can find out.