Nigel i have just brought a invicta inset stove and i am fitting it were my Gas fire is could i use the same flue or not also the cap on top
No you cannot use the same flue. You need a multifuel flue. Have a look at this site > http://www.fluestore.com/index.php .
You will need :
Flexi Flue
Flexi flue to vitreous flue connector
Sleeve for the inside of the flue at the top
Rain Cap
Register plate
Top plate
Top clamp
On saying all of that, if you have a sound, leak free chimney with a decent draw you will just need a register plate and vitreous flue, and remove the existing flue. Get it checked by a HETAS engineer.
EDIT: Sorry, just noticed you have an inset stove, you will need a flexi flue, although you wont need a vitreous flue section.
Box clever
Thanks for the advise and the help.