Thanks all. My motor starts readily on a cold morning , with choke, but takes a while to warm up. So I have to avoid a situation where i need a lot of power quickly. Then cold- starts mean wear........I have not had a reply from Rotax (so much for their after-sales-service!)Perhaps i will get a reply from Pipistrel? They advertise that kind of flying , with a Rotax.
Rotax offer a thermo controlled flow valve for the coolant - not cheap - but probably the answer.