Problem solved, I'm selling the Roland Plotter and have returned the A3 Canon BJC800 printer to the supplier as faulty.
A couple of days ago I took my newish PC into PC World for a (free) checkup and had a look at printers to try and identify one that I might be able to pick up on ebay.
I was surprised to find a new Brother A3 & A4 printer for £62!! The A3 printing means feeding in a single sheet of paper at a time but that's fine as I don't need to do lots of printing in A3.
I printed out one of my drawings made with DesignCad and checked a dimension. It was spot on using a steel rule.
I'm grateful to all who answered my questions, they may help someone else but for now, all is well (apart from the cost of new cartridges for the Brother that is.)
Edited By Brian Hutchings on 29/01/2016 10:55:00