Just to "round off" (sorry) the turning block issue…..CMB actually received the afore mentioned turning blocks and have sent them on to me. They are 20mm and made of aluminium and brass.
Krick 'did not have them listed' -as I mentioned earlier- because I used the wrong part number when searching! They came in Caldercraft-labeled packets 3 in each for just under £9/pkt.
I don't know if ALL below-decks running rigging actually needs to have long spring-tensioned runs fitted. Maybe this something which is only needed for the racing yachtties? I have seen some installations with ENCLOSED drums on winch servos and no tension system applied to the lines. Two of my 5 winches, one Krick/Robbe and one Graupner have enclosed drums. Don't know if that's good, bad or ugly. I expect I'll find out around Easter.
Thanks again to those who replied with info on possible sources.
George