i would honestly say that no you havn't!
I had a cambelt/balance replacement done once and the mechanic told me no need to replace variator as it
wasn't necessary and a month later the diesel sound began
Now the reasoning is that its not a good idea to intall variator at later date as belt should not be retensioned and a new cambelt is necessary if variator is necessary! idlers etc should all be done at same. As for waterpump, if the impellers were plastic the natural upgrade when the cambelt is been done is to replace with a new one with metal impellors is advised and good advice!
As for the cam cover gasket, i would expect to replace this as the head would have been removed to do the cambelt and lock the cams if the job was done properly, so a new gasket is a matter of course and the obvious give away of a failed gasket is oil in the first two plug holes! These will effect the plugs and may deem them necessary for replacement, the only thing i can recommend is ask for the plugs they remove and examine them yourself. Th advice appears to be good advice by the garage, however! the expense will be considerable!
ball park cambelt, tensioners & idlers £350 - £500
Cam Gasket £15 half hour to fit in good garage
Plugs £60 -65.
So, your service looks like a biggie
ball park £800 - £1k, But! the end result is that you will have an engine that will be like new and last at least another 36k mile so long as you maintain basic regular maintenance!