Err... Sorry on closer inspection I found some pre drilled holes in the prefab concrete walls roughly where i wanted it.. And I'd bought a few different diameter rawl bolts in preperation... I had a couple the right size... Araladite on the rawl and on the thread of the bolt...
So all sorted... With big blob of p38 on bolt heads to prevent removal with socket...
Sorry no entertainment there folks

.. Just wished id spotted the holes 2 years ago..... I assume they were part of the
fixings of the walls that weren't used