I'm not in UK, but used to be years ago. Contact B&C sprockets. www.bandcexpress.co.uk - You may be able to order online.
You should have no problem getting an alloy anodised 41 rear sprocket and 15 tooth front from them. Also get the best RK chain they can offer.
Don't be tempted to do a 520 size chain & sprocket conversion, as that means less strength for negligible / bugger all performance difference. Stick with original specification & stronger 530.
Have seen two examples recently of people with smashed casings on RC51's from broken chains. It's not worth the risk or lower mileage life of 520 IMHO.
Renthal is based in Manchester.
Should be super easy to buy a set of sprockets in any tooth number.
I've been running a Renthal 15T front and Vortex F5 41T rear for over 2 years now.
I bought the Vortex since I have Marchesini wheels, which don't use the OEM bolt pattern.
I installed an RK XSO chain with the set and they're lasting incredibly well.