I haven't actaully done that route yet.. I usually head south east and go further into the Peak District, there's some great routes around there.
I've bought a few things from Evans (Deansgate), like a helmet and gloves. They were alright for things like that but they're quite expensive. The plus side is you can try before you buy, and if something doesn't fit you don't have to worry about sending it back. If you buy a bike from them have a look through their catalogue/website first because they only have a small range of their bikes instore. Might find a better one online.. they can order it in if you do.
Again with Decathlon, I've only bought smaller things from there, like water bottles, bike toolboxes, pedals, etc. I really like them though.. good value for money. I don't know much about their Rockrider bikes but my mate says they're good quality.
What's this hybrid that you've got at the moment.. would you consider upgrading it? With £300 to spend you could make a lot of changes to it.