Ideal Standard/Armitage Shanks (same company). Graham merchants does a kit with a new washer and plastic plug to hold it in place.
Note that it is VERY important that there is at least a little free play beween the flush mechanism and the flush buttons on top of the cistern lid. Other than that, I think this is a good design of mechanism and worth keeping. As you say, you can also buy a new one and swap the bodies over.