My bath taps are extremely hard for my kids to turn off. I know I need
to replace the washers but I've resisted doing it as I can't seem to
find a way to turn off the water except at the mains - The hot water
continues to flow as does the cold from the tank in the loft. What are
the sizes of washer I'm likely to need to do this? If I'm going to have
to drain the hot and cold tanks I'd rather have what I need available
without having to go out in the middle of it all. The taps are about 20
years old if that suggests anything.

From your description it's an oldfahioned unpressurised system, like what
I've got.
There may (should) be a valve between the hot outflow and the hot water
cylinder inlet.

It probably has a wheel as a handle rather than the cross bar of the
mains stopcock. If you find this in the airing cupboard or loft and can
turn it off it should stop the hot flow pretty intantly, though the tap
may well continue to drip a bit - ignore this.

Another way is to block the outflow of the cold tank with a bung, this
may stop your cold tap - it should really come from the mains pipe these
days; some folk say a carrot is good; if I had to do it I think I'd use a
good quality rag or summat - you don't want to end up blocking the pipe -
but I'm speculamating as I've never had to do it

HTH

mike

If your tap hasn't been touched for ages you may be in for a problem
stripping it down, so be prepared.

mike