thanks for all the comments folks. I considered all and decided to take the learning curve. I was lucky the transformer was only held togethert by a non sticky clear plastic wrap.
Having unwrapped the windings I took the 55v secondary off completely. Re the 20v secs, I worked out the value in volts of each turn, then removed 8 turns from each of the 4 secs. I now have 16.48 volt (near enough to 16.5 volt for me) I had to re-space the secondary windings (i did it to look neat - there may be a sound physics reason too).
Lets hope it works. Before the 'surgery' the 20 volt secondaries were good for 18amp in parallel (according to manufacturer). Tim stated that lowering the volts to 16 will give 25% more current. so I should be good for at least 20 amps. I only wanted 15 so the margin is ok.


thanks again folks Andy