I have a yamaha tdm900 2002,bought it about a year ago,only thing wrong it is lumpy around town,I was told of a mod to sort this out disconnect breather pipe and seal end to engine of course it has blowen the rocker cover gasket semi circle seal at side of engine,I have reconnected the pipe but was wondering is this a problem with the bike or was it just me being a numpty, is ir a big job to renew gasket, anyone?

the mod is actually to disconnect the vacuum pipe where it connects to the airbox, not the engine breather! The vacuum operates a flap in the airbox which will be disabled when the pipe is disconnected.

you've over pressurised the crankcase and it's blown out the weakest point.

it's not a big job to replace the valve cover gasket but you do need to have some mechanical knowledge.