Hi there,
I am not much help really, I had the same trouble and couldn’t find the details anywhere. I eventually tried various belts until I got the right ones.
i sold it in the end and when the guy came to collect it, it through the drive belt.
It was a pain
sorry.
david
I just looked in my old eBay purchases and I think they were 2 x B33 and 1 x B43 belts, if that sounds about right then they are the ones I used.
I also remember that they were a pain to fit as you need to depress the spring on each cutter arm to get them in place. That is probably why they are slipping as the previous owner had that trouble. I used a length of thick twine and put it around both the cutter arm and the chassis and then wound them up with a bar until the spring had compressed enough to get the belt on.
have fun.
david