i wouldn't trust kwik sh1t to put dustcaps on a valve; with the power of google i came up with these quotes from here http://www.sheffieldforum.co.uk/archive/index.php/t-158883.html
Bartram Motors, on Olivet Road at Woodseats. It's just a little two man business in a shed ... but they've always been very efficient
Gordon Road Garage, near Sharrowvale Rd. Hunters Bar.
D & L Autos off Rutland Rd have always been good
Try Guernsey Garage on Guernsey Road, 0114 2580615. It's at the back of Heeley Bottom, arcoss from Dunhelm. Alan's the mechanic. Totally reliable and very reasonably priced. I've been using him for 10 years now and wouldn't go anywhere else.
Have also used Snuff Mill in the past and only have good reports.
D.G. Autos who are about a 5 minute drive from Banner Cross.
They are at the end of Abbey Lane, on the outskirts of Woodseats and I have used them for years. Very honest, fair and competent.
0114 274 6472 is their number