Thank you guys and for all the suggestions and the links
I didn't think of googling for bicycle trailers, there are also a shed load of them on Amazon, see here:
https://www.amazon.co.uk/s/ref=nb_sb_no ... le+TrailerCompared to the price of mobility scooter trailers, they're virtually identical and a lot cheaper. I might have to modify the tow arm to fit my scooter, but I would have needed to do that with a Mob Scooter trailer.
The mob scooter ones are designed to hitch/slide into the 1 inch square tube under the seat. But it's a bad way to attach a trailer as the weight of the trailer pulls the seat about as it's only mounted on a centre seat post.
If I can pick one up for about £50 it's not worth the hassle of trying to make one.