my goldfish is about 17cm long and about 20cm round, (mostly belly)he was like this when we had it given to us;

Last week he suddenly was upside down and not swimming, every now and again he turns upright for a few seconds and then back to upside down, please help;

Please don't feed the fish for a few days and then feed defrosted unskinned peas.

Aside from the great advice already given, you could try buying some Epsom salts, available at any pharmacy You can then add a dosage of 1/4th teaspoon per gallon of water. This works as a muscle relaxant/laxative for goldfish and should also help, aside from the water change, fasting and pea diet...

As it's already been mentioned swim bladder can be caused by a few things.

Do you know what your water parameters are Lisag? I.e. ammonia, nitrites and nitrates?
You can either take a water sample to a store and get it tested, or purchase a liquid test kit to use on your own at home

It could be a water quality issue, so by testing we can rule it out.

It's far too small for two goldfish, for a moor, it would barely be enough but he'd get by.
Here's what you should do:
- Go to the pet store with a sample of your tank water, say a cupful or so, ask them to test pH, Ammonia, nitrite and nitrates (THIS IS IMPORTANT!!) or buy yourself a test kit for these parameters, you'll need one eventually either way.
- Perform an average water change (say 50-60% of the tank volume), vacuum the gravel and add a water conditioner and beneficial bacteria (such as Prime by Seachem or similar products).
- Don't feed the fish anything for a few days and later feed them only de-frosted, skinned peas

This should by all means help, unless the fish has something more severe going on.

Could be that the little guy has some chronic swim bladder issues for whatever reason, so you need to feed him mostly vegetables from now on, and change out 50-60% of the water every week if you want to keep him healthy