Just remember that the rear wheel supports 75-80% of the weight of the team. So if your team weighs 300 lbs (about 21.5 stone given your spelling of "tyres"), the rear whell needs to reliably support about 225-240 lbs (about 16-17 stone). That would be a Clydesale/Athena rated rear wheel.
Cheers!
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2008 Red Co-Motion Speedster Co-pilot (Redster)
2009 Surly LHT (captain's commuter)
2009 Surly Crosscheck (stoker's road bike)
2007 Giant FCR2W (stoker's commuter)
1980's NOS Legnano (stoker's toy)
1970's Stella rebuilt as fixed-gear (captain's toy)