I seem to have finally solved the problem (chattering and squeaking):- Finally got some Lidl W5 all purpose cleaner (better than Flash in my experience) made up 8:1 solution and with an old but clean dish towel gave both screens and all wiper blades a thorough scrub. Quite surprised how black it all was although not obvious from a close examination. Apprently it was probably rubber dust from the road according to the American tipster who I got this tip from in the first place and it's the thin film of same on your screens and blades which causes the chattering, etc. I must say how smoothly and quietly the rear wiper now moves compared with what it did before cleaning