Sorry, I do not know anything about this plant. Have you tried looking it up on Google, or RHS website, they usually have helpful information about plants and how to care for them? That's where I get mine from. Good luck!

Yes, you can give it a good cutback, but wait till May, preferably after it's flowered, don't do it now. You could, if the weather's not too cold, do it in April. Unfortunately, though, if you have not been cutting it back on a yearly basis, you will now have lots of very old wood, and cutting it back to that might mean it does not regrow. If you can't stand the way it looks, you could chance it, after taking cuttings, and make sure you prune any replacements when they should be done. More info in the link below

http://www.helianthemums.co.uk/cultivation/pruning

Thanks Michaella, I did google it but I didn't really find enough on pruning. Maybe I was too impatient!
Thanks Bamboo for your advice(no I havent pruned it every yr cos it flowers for such a long time & then can't make a decision!)I shall see how the plant & the weather is in April & make a decision. I wonder how it will react if I prune half before flowering & the other half afterwards! I'll have a look at the website too.