Oh there is a brilliant man called Dermot.
His number is 07961 090219.
Many thanks Marian, I will give him a ring.
I called Dermot and can certainly recommend him - not least because he came on Saturday morning, as agreed, checked the 2 windows and he said our sash cords were fine and would outlast the windows (or possibly us!) and wouldn't take any money for coming out which I thought was very decent as he could have replaced 4 cords, charged us, and we wouldn't have known any better. An honest workman!
He doesn't just do sash cords needless to say, he is a glazier so cuts and fits glass and mirrors.