Ok, i will reply to this myself since the problem has been sorted.  I didn’t get any replies so I went onto Don Ashton’s pages and with a few tries and lots of reading I used his spreadsheet to design the valve gear.  I discovered that very small changes make very big differences so one lesson is that the parts have to be very accurately made which I didn’t appreciate.  Also it’s possible to try subtle alterations to get it to fit into the design.  After this I used the Wallace simulator which Don references.  Again it didn’t turn out right to begin with but I found that by adjusting the valve setting by just 0.020 it all came out right and the valve events were really good.  So to anyone else who fancies changing a slide valve engine to piston valve i can thoroughly recommend Don Ashton’s website to design the valve gear.  I also think it would be no bad thing to put the dimensions into it of any valve gear design you are building, many of these older designs are questionable.