I bought a model-specific tailored one this year from Protec, it's simple to fit, can access the gas locker easy enough, legal as it has clear panels to let the front marker lights be seen - and importantly, keeps all the crap off the front of the caravan!
The other main manufacturers is Specialised Covers but theirs seem unneccessarily over-styled, complex and expensive.
I think both companies will do a good deal if you want both a winter cover and a towing cover.