I managed to get it on in the end, basically scraped away some of the crap between the cogs and gently tapped the cassette down with a mallet. The lock nut then went on fine, thanks for the advice 🙂
Step 2 lol – setting the derailleur up, having re-fit the wheel and aligned the H screw on the smallest cog I manually moved the derailleur to the biggest cog (smallest gear) but when adjusting the L screw it wont go out far enough to align with the cog. Have I buggered something up with the cassette?
Sorry for the basic questions, after years of using bike shops for work I have decided I should learn my self. Can be a pain in the arse but it will pay off and is rewarding.