We don't fuss "old thread" here. It is better to keep info in the proper post, regardless of date.
Leaking lids are a common problem w/ old cast-iron MC's. If you can correct a "fluid geyser" problem, that helps, but a good lid shouldn't leak even with splash. If it leaks fluid out, it lets moisture in, and that causes internal corrosion with normal glycol fluid. I understand there is some humidity in the UK. That is one reason to change to a modern aluminum MC w/ plastic reservoir. But you will lose that traditional look of a rusty MC and rusty firewall from dripping fluid (great paint remover). If you prefer 1-stop shopping, Dr. Diff sells a package of 2-4 bolt adapter plate and MC, the later for a 1980's Dodge truck. If manual brakes, insure the bore is small enough for your leg, especially if you have front disks. Professional football players ("Rugby" for American colonists) can get by with a larger bore MC.