Thanks everyone for their replies. I've had a little play around but have not been able to sort it. To make sure I don't mess it up any further I have now found a shop I can take it to. They will have a look at it and hopefully this will sort the issues.

Thanks, Hal. I had already changed the feed dog for a new one, as the old one was worn. Unfortunately, this hasn't made a difference. Changing the pressure doesn't seem to make a difference either. Any further help much appreciated.