I think that you need to check your documents again,
£50 excess seems a rather low amount when it comes to insurance..
You probably find somewhere in the small print mention of 'the insurers minimum excess' and the insurers minimum excess will if more than your agreed excess overrule it..
I got caught out last year on this will car insurance, amongst several other little, small print didn't tell you!
I had full comp on my car, that I had a £100 excess on (I thought) and a courtesy car provision if my car was in an accident...
My car was written off last year in mysterious circumstances (as in nothing I know about the incident makes sense what's so ever) My policy clearly stated in the main part details, that excess was £100 also I was entitled to a courtesy car!
In the small print buried deep, was a mention that the insurers min was £250 and that the courtesy car would only be issued if the insured car was less than 6 years old..