Well thanks for the advice guys😀, using a rotary tool i got from Lidl with what looks like a tiny circular saw blade it cut out just great ,i used masking tape and drew the cut lines where they were needed ,it worked best when not running the tool too fast as this just melted the edges and i ended up welding the join back up !. so slow and easy made it a joy to do , and no cracks ! 👍
Next on the to-do list is find a motor, the plans call for a "540 motor", when i put this in to Ebay there are a wide variety, any good reliable brands or are they all the same ? Tia

Colin & river rat, thank you, I had exactly the same question ( I hav a BILLINGS African Queen & a DUMAS military tug, both with ABS hulls. This site is a wonderful resource for newbies like me😎

I agree with Colin, a rotary tool with a cut-off wheel on it is your best choice. Just be careful as they can slip and cause damage. Tape will also help. Sand paper cleans up the edge nicely. Hope this helps, Dave

Mark I have tried, extremely sharp scalpel, Stanley knife, scissors and even razor saw.
But the only way I have managed to cut without splits is to use my rotary cutter (like a dremel).
One tip is to apply masking tape and cut through the tape.
This is my way, other's will possibly have a a better one.
Cheers Colin.