From this page it looks like your computer has a serial ATA (SATA) connection for the hard drive. You should be able to purchase any size laptop drive that you want.
Depending on the age of the machine you might want to consider upgrading if the rest of the specs are similar to that machine.
Thanks for the advice that's good to know.
The laptop was bought from a Microsoft re-manufacturer or whatever they are called so has Windows 7 rather than Vista which is what it had previously according to the sticky label attached to it.
It works well but the HDD is a bit small by today's standards but I didn't know if there was some mobo or other factor that might
stop a big HDD being fitted.
All I need to do now I believe is buy a new HDD, a USB powered caddy and find out which software will "clone" the entire HDD including W7 so that in the event of a major fail I have a HDD fully ready to go as a spare.
(I will ask for advice about cloning in another thread on the forum)
The difference in price between a 500gig and a 1tb is not a lot now so might go for it!!
Many thanks for helping me with this query.