Try N & J Mussett Mersea. 01206385658. http://www.nandjmussettsails.co.uk/
Only used them myself for cleaning but have seen their work and I am told cost is reasonable.
Worth getting a quote from Sail and Cover at Woolverstone as you are at Pin Mill.
Tel 01473 780075.
They have made me a new sprayhood and dodgers which I am very happy with. These are now beige so when I get a pic of her in the new livery I will update the avatar /forums/images/graemlins/grin.gif
Nick & Jane (& Adrian) Mussett have done all the canvas for our last two boats, including cockpit tents etc. I would recommend them very highly, and their pricing is pretty sensible.