I am assuming, Dindsy, that you have now done the
outside test?
If so (and in no way manner or form did I mean in my previous post to counter
Iceman's comments regarding collimation) and you still cannot attain a decent focus then collimation is probably the solution.
Please, do not be put off by the 'jargon' used in astronomical fora such as this. Collimation is simply the lining up of a telescope's mirrors or lenses. It is not hard,
but it can be fiddly. You do, of course, need to get a feel as to what this means.
If you are not fully cognisant of the ins and outs (or the jargon) I recommend that you view this:
http://www.andysshotglass.com/Collimating.html
"this" is a short (but almost 4mb) explanation of collimation. If you can afford the bandwidth it's well worth watching.