Since the plumber connected my washing machine drain to the sewerage pipe in the garage when I flush the downstairs loo it is gurgling and only leaving half the water in the trap. Is this because you shouldn't connect a washing machine to a sewerage pipe.

What exactly are you calling a sewerage pipe? What else would you hook a drain to other than a sewer pipe? Please describe the plumbing hookup in more detail, maybe with some pictures. Sounds like something wasn't vented correctly. Did they by change hook the washer drain into the toilet vent? If so, that's a no no. It is possible that the new washer drain has nothing to do with the toilet problem, but its hard to say without more info.

Not sure how or what he did but it seems he added an unvented drain that can siphon water from your toilet trap. Either you need another vent or the one you have is clogged. Can you get on your roof this time of the year?

I have a sewer pipe from the upstairs loo which comes down through the garage which is where the washing machine has been situated. The sewer pipe is black and around 6" diameter. He has attached the drain from the washing machine into this sewer pipe. When I pointed out the downstairs loo malfunctioning he said we needed an extension on the vent pipe or it could be blocked but he does not do that sort of thing. I am reluctant to pay his bill until I know that what he has done is in fact legal and I won't have to pay again to have the washing machine plumbed elsewhere.

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the plumber connected my washing machine drain to the sewerage pipe in the garage

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How did he connect to the sewer line? Did he use a trap? Did he run a vent from the trap? If none of the above were done the you have a illegal connection. Let me know, Tom