Most mechanics do not clamp at the top tube. Some of them are very thin.
I always clamp by the seat post. If the bike has a carbon seatpost I will fit a cheap alloy one to clamp it. Get a a stand that clamp this way.
£50 is always going to buy a stand that is comprimised in some way. I wouls spend a bit more if you can. Kestral Engineering make stands like this.
http://www.kestrelcyclestands.co.uk/bic ... duct_id=56
For me I had a hot rod builder who was a downhill racer at one point to built me a stand entirly from steel. So I have stand with a 3 pronged base that over 4 ft in diameter for stability, a massive steel post to which the clamping arm is attached. The clamping arm can rotate in two perpendicular planes and be raised up aand down. The clamp itself is steel and closes by a a screw so I have complete control of the clamping force. I don't have pic but it cost me £300. It is over engineered but will last a life time. Park make a similar stand but it cost twice that at least.
I am sure a cheaper version could be built that is smaller and have less adjustability. That way you will have a stand that is stable and will not break.
ALAN Competitizone 1980-1981, Traitor Exile, Pinarello Monvisio, Trek 2300 (the one with the carbon tubes), Scott Pro Racing, Vitus 992, Kinesis FF29, Sannino, Trek 8900, Genesis Equilibrium Ti Disc, Gary Fisher Ferrous single speed, 1948 raleigh record ace project, Klein Quantum, Ceilo Rosso TT bike, 1962 dawes unknown model.http://www.thecycleclinic.co.uk