If you are in Ely (I am in Swaffham Bulbeck just down the road) then Halfords in Ely has the Muc Off wet chain lube. Comes in a spray can.
Things to do:
If you can remove chain, then remove it, put it in a big jam jar, spray with GT85, rattle it, then pour in about 5cms of Muc Off pink cleaner OR hot water with a good squirt of washing up liquid, plus some hair shampoo. Then lid on and rattle lots. Rinse off with lots of HOT water and dry in sun quickly or use hair dryer. If you twist chain there should be no crunching or grinding if you twist it.
If you can’t remove chain, then place a rag underneath it and spray with GT85 and run it through and through the rag, by holding rag in a bundle around the chain and turning the cranks.
To apply Muc Off wet chain wax, hold a rag underneath the chain so that it doesn’t go everywhere and GENTLY trickle-spray the links as they pass over the rag. Look at the chain as it passes and do it in sections, so each link gets some. Leave to dry before riding - around 15 mins.
I ride in Thetford a lot, which is incredibly sandy - really abrasive - yet through this cleaning and lubing regime my chain and cassette and chainring have lasted thousands of miles there - currently five years of constant riding.
