We use MTM Herman who were recommended to us. So far they have replaced two sash windows - one entirely including the box frame as it was rotten and the other just the sashes and have refurbished the frames.
They are very reliable, clean, trustworthy and fairly priced. Their website is www.mtm-herman-joinery.co.uk. Our contact is Mirek 07885 551724.
I have just had 3 sash windows replaced and would highly recommend www.mtm-herman-joinery.co.uk They have provided a bespoke service to match exactly the design of my existing windows and replaced them with a fantastic double glazed version. Excellent craftsmenship, reasonably priced. Very happy customer.