Hi Jak,
Do it yourself! It's really easy. I've re-covered a number of seats, and the TRX was the easiest yet. A person with your skill's won't have a problem.
You do need to be able to get hold of a staple-gun (for upholstery, not building!) that will take a minimum of 1/4inch (6mm) staples. Remove the existing vinyl (using a small screwdriver, or a nail-file! 

 ) and take that down to the upholsterer's to get the required qty of vinyl. You also use it as the template. The existing vinyl is really thin crap and will have hardened up considerably (I'm sure there's hundreds of purile mind's out there thinking what I'm thinking!! 

 ), so allow an extra inch on the sizing of the pattern. You can trim this off after.
Start with the straight side's, then the rear and front, using the staple gun firmly. The plastic base is fairly soft, and the staples go in reasonably well. Once the straight sections are done, gently stretch the corners into place. If the vinyl wrinkles, remove the staples and approach from a different angle (slight pause, to allow for more purile thoughts)!
Give it a go. At the most it will set you back 5 pounds, and you'll have personalised your bike!
Bugger! I've just re-read your post and you're talking about the small pad! 

  Same process though - the only difference is you need to trim the folds to reduce the build-up of material (same as you would when hemming a seam). 
 
And use lots of staples!
 
			
			
									
									These are my principles and if you don't like them...... I have others.