I think it's generally fine.

The only thing that particularly looks bad is the images being stretched and not the best photos in the first place. There are modules for oscommerce which resize within constraints rather than to a fixed aspect. Also may be worth adding lightbox or similar to allow zooming in on the images.

When I go to checkout I'm presented with a login box, but the link for new customers is fairly un-obvious - "By creating an account at UK Parts Direct you will be able to shop faster, be up to date on an orders status, and keep track of the orders you have previously made." is unnecessary waffle. Reduce it down to a 'New Customers Click Here' sort of thing. I would consider allowing anonymous checkout, actually.

Mobile number: 0787 88086096. Since when are mobile numbers formatted like this? 5-digits then 6-digits. Mucking about with it just looks unprofessional imho. As said, a VoIP number would suit you well. It can be local or 0800/0300 etc and isn't too expensive. See sipgate, pbxes, or gradwell.

Your links are all the same colour and style as the body text which essentially means they are invisible. E.g. 'Forgotten password' link.

Otherwise good on you I moved away from oscommerce because I was finding it a nightmare to add functionality too so keep it simple if you dont want to go insane!

Chris