Suggest going on line, plenty of Gaslow providers, costs vary between size of bottles you want, stainless steel hoses, and external fill point. If you are confident & know the LPG regs you could carry out the install yourself, otherwise get Vanbitz or a dealer to do it for you.
Averaged cost for 2 11KG cylinders, all hoses, external fill outlet, budget for £500 -£600 (but check with Vanbitz for an accurate quote), Give them a call,or PM @eddievanbitz on Monday they are great guys to deal with, peace of mind, great quality installs. Get10% Funster Discount, if you mention it upfront.
Edie of Vanbitz is not in Bangor, they are near Taunton and have their own lovely campsite that you can stay on overnight, fitting the Gaslow Kit shouldn't take them very long however.
Autogas in Thirsk, are also very good, as they offer the Light Weight Aluminium Bottles, as well as underslung Tanks, that can be very useful as they free up the gas locker space to store other things.
Hope that helps.