although the cistern is full , it takes quite a few goes for the toilet to flush as the flush handle is loose for the first few (usually about 10) attempts. Is it a siphon problem, leakage? Do I need a new cistern or can I buy and fit a part (ie siphon ) to correct it?

new syphon or diaphragm.
from a diyers side it's easier to fit a new syphon than hunt around getting a diaphragm.

Does sound like a new siphon / siphon diaphragm is required. If its a high or low level cistern than its pretty easy as you can access the backnut holding the siphon in place ( obviosly empty the cistern fully first) . If its a close coupled cistern then this is more involved as you will need to remove the cistern from the pan to get at the backnut and will also possibly need to replace the donut washer between the pan and cistern.
Also check though whether you have got a low maintenance 2 ( or3) part siphon with a collar / pin arrangement which allows it to be separated in - situ ? If you have then it may be easier to go down the route of replacing the diaphragm only.

Good luck and keep us posted.

Thanks for the help. It is a low level cistern . Would I need the exact same siphon as a replacement or is there a generic type I could get. The make is Fordham 1305 (seems quite old). I am not aware how to identify collar/ pin etc so a replacement siphon may be easier for me.

I have dpne a little resaerch and decided to replace the siphon (looks very easy) with one from screwfix @ £4..99 Thanks again!

you most probably won't need to change the siphon just the diaphragm - this is the platic bit at the bottom/inside the siphon.

You can make one from a strong thick plastic sheet / stong thick plastic bag type stuff or buy them for pennies.

I changed the diagphram and kept the old siphon as you say. I used a laminating pouch which worked perfectly.