Hi johnboy, and welcome to TGMF.
Not one of my favourite machines to be honest, but even so £400 seems very steep for a service on this type of machine, especially if the chain and cover are the only extra parts required.
I have had a couple of chain covers which had worn through, but usually associated with running along the edge of concrete paths etc. They really need a protective steel strip welded to the frame below it. I also had one that broke the roller shaft in half, that also suffered chain cover failure.
Belts, the last one I repaired had fairly loose belts, and this was caused by the double intermediate pulley which had lost it's spring tension, the spring had gone very limp.