how to look after a lavatera in winter

No, absolutely not. I'm assuming you mean the woody type of lavatera that has a woody base and is a deciduous shrub. Wait till April, then cut it right down to the base - if you're in the north and its still cold, wait till nearer end of April. You can, though trim back longer growths now if its in danger of experiencing wind rock over winter.

Definitely - I lost one through wind rock. :-(

My laverteria has grown to about 12feet in height this year so no way will I leave it at that height all winter.
This week I will reduce it by two thirds then next spring, when it is starting into growth, I will reduce it further.