Hi Graeme,
I don't know why i woke up thinking about this, but i did. I had another thought.
Maybe your finderscope mount is fine. If your primary mirror (Astro lingo for the big one at the back) is grossly out of alignment, (could be with a new telescope bought from a non astro suppliier, or be dislodged in transport ) then it will be gathering light from somewhere not parallel with the tube itself. This will make your finder scope difficult or possibly impossible to line up.
Read up on "collimation". There are great articles on Ice in Space, but before i discovered IIS i found great comfort in the attached article. It is a good beginner's read:
http://astronomy.concreteairship.com....htm#alignment
I discovered simply with a
plastic milk bottle top with a hole in the middle in lieu of the film canister described on the link that my eBay scope was significantly out of alignment. It was probably never collimated in the first place. I was able to make great improvements in "seeing" just by adjusting by the hole in the plastic cap method. I later splurged out on a laser collimator.
Check it out with this basic method first!
Michael