I notice that in April issue 93 of the Macgregor Sailor there is an article on the ballast valve on the 26M. Our 26M Tarka had a problem with the same ballast valve - the bolts had corroded and broke when it was removed whilst preparing for Coppercoat antifouling treatment. Apparently the valves used on the 26Ms are made by Valterrra a US firm and are originally designed for use on American motor homes as a waste valve. Repair kits containing new seals are available for about £10 on eBay and the whole three inch slide valve is not particularly expensive if sourced on the internet. See attached photos.
Incidentally the Coppercoat antifouling treatment is a great way of preventing weed growth on boats that are kept afloat for long periods. It also helps protect against osmosis as it consists of a heavy loading of copper powder in a thick epoxy layer. It's not cheap as it needs to be professionally applied but works out no more expensive in the long run than good quality antifouling. It also saves the pain of lying on ones back under the boat whilst sanding off old antifouling and applying new coats at the beginning of every other season or so as the Coppercoat is guaranteed good for at least 10 years. There are boats in our marina that look good and are still weed free after nearly 20 years.