When I empty my bathroom sink or flush the loo water comes up through the shower plug hole? The loo also fills up quite a bit as does the downstairs loo?

Sounds like its backing up. You probably have the sink waste pipe going into the shower waste pipe. I would run a seperate waste for the shower, basin,into the soil stack, and check out your soil stack for your loo, sounds like a possible blockage.

If it has just started happening i would assume you have a blockage somewhere,could be a open vent problem,hope that helps nick

it seems that your soil pipe might be blocked

Difficult one to pinpoint without actually visiting. However if the you are getting water coming up through the waste of the shower when you either flush the toilet or empty the wash hand basin, then it sounds like the plumbing of the waste pipe for the shower does not have a sufficient fall (gradient) on it to stop back flow. You may even have a blockage somewhere if your toilets are backing up when you flush, get somebody out to have a look. A decent plumber will soon be able to ascertain the problem and find a solution for you.