As no-one knew the answer to these questions, I'll pop up what I found as it might assist others.

After a lot of digging it appears there are 'standard' and 'slow' belts for the 550 Pro - CH85667A and CH85678A being their offocial part numbers respectively. For the 'standard' belt the internal circumference is 914mm, which is 36". Now I have no idea how these are meant to work as I mentioned I literally could not get the idler pulley in with how tight a M36K belt was. The ODK store says these are for the 550 Pro in the 20" deck....so perhaps there was 550's in the 20 and 22" decks? I have the latter.

So the 'slow' belt has an internal circumference of 940mm, which is around 37". But as I said when I tried a brand new 37" belt, which was a Carlisle Blue Wrapped it was way too loose. Just retested and thats still very much the case.

So then I found a supplier selling what they what they said was the genuine Victa CH85678A belt, which was now a Gates 6738 Powerated belt.
https://www.[Censored].com.au/v...ted-ch85678a-85678a-ch85678a-85678a.html

From looking up this belt on the Gates catalogue one finds that it's an M36.5 belt and is specially made for rear tensioned belts - so would work well in the 550 Pro.

So in theory that would kind of explain why the M36 was too tight but the M37 was too loose. Anyway hope that helps, I've ordered one and I guess we'll see how it goes.