Mine has fallen off sometime and I only have some sticky tape covering the gap now. Is it for the prevention of dirt getting in and will it be very detrimental without it?
Where do fellow UK riders order their parts, and do Suzuki take orders direct?

Regards

Don't worry about it, most of the crud comes up off the disk anyway, rather than from the top.

What you DO have to worry about is the two steel caliper retaining pins.  Unless you pull these out and grease them at regular intervals they will sieze in their galleries in the alloy caliper.  This makes rear brake pad removal a life changing experience rather than the quick easy job it should be.

What about the rain and all the muddy crud and stones off the roads? I hope the dealer doesn't make me buy a whole caliper.

Look in MCN ads for breakers - usually loads of GS500s being broken.  Should only cost a quid or so for a used bit of plastic.  For new, try Robinsons Foundry (search on Google).  You won't need a new caliper.