Yeah I know how to get to the hop up
Unfortunately Im not in the UK currently. Im working in Brisbane Australia for a couple of years so airsoft is denied me for awhile 
Now im not 100% positive as Im relying on memory but you might be able to remove the barrel with hop unit wthout actualy opening the reciver. There are 2 screws on the underside of the lower barrel support (just in front of the magwell). These are probably blocked by the spas cage. Theres also a very small hex type grub screw on top of the barrel that also fastens it to the receiver. I think once these 3 screws are removed that the whole barrel with hop unit will come out.
As for replacement parts in the UK then I can personally recommend Zero 1 airsoft. They might have replacement in stock already but they can order in just about anything. They are very reliable.
Another retailer is Airsoft Dynamics. They also can get all the bits but delivery can take awhile as they only put orders in to hong kong either fortnightly or monthly.
You could also find out where your local skirmish site is and find out who the resident 'armourer' is and see if they will fix it for you.
Phil
theForce is my ally, but a kick A$$ lightsaber helps........