hello,I have two std hibiscus about that size in my garden and i always wait until the last frosts before i prune them, this is usually about the second or third week of may where i live, the odd frost we may get after that does not appear to have much effect on them,I usually cut back by about 2 to 4 inches and remove any branches that are trying to grow inwards or across another branch this will keep the shape you are after,if you look on the branch you are pruning you will notice small buds i then cut about 1/4" before it, this may not be the correct way but it has served me well for over ten years,
talking of frosts we had a severe one this morniing, winter is hanging on,
hope this is of some use to you,
ogon.

Thank you very much. I shall follow your advice which is much appreciated. I live in the south west of Wales and we had a slight frost here this morning aswell. However the day has been glorious.
All the best :o)