How much is reasonable to pay for new plugs and cambelt on top of a B-Service for a '98 GS?

My garage have quoted £ 220 for the plugs and £ 299 for the belt. All inclusive of labour, parts and V.A.T.

I thought that much of the dismantling work was common for these two jobs.

Sounds way over the top.

if the engine isthe same as the mk1 then you have to take the throttle bodies off to get to the plugs. So you will be charged about an hour for that then the plugs and the cambelt. You are looking between 80-120 for the cambelt change, probably 35-40 for the plugs from the garage and the rest is the cost of the throttle body removal.
I'd do the plugs myself if i were you it isnt hard and the ik16 plugs are cheap from the US ebay