Hello again & a Happy New Year
Tom, MarkT, yes its a board like this
The solar panel does not power the Arduino and related parts, the power is provided by a 9v 2a transformer, the same transformer that provides the alternative power to the pump.
The solar panel only powers the pump when its providing sufficient power to do so.
My original question (i tried to keep it simple and focused) is mostly moot as I've changed the circuit so that only the transformer supplied 9v goes through the LM2596 now.
The solar panel provided 6v is now not trimmed and in full sunshine will go over 6v (not really the time of year to stick a meter on and measure by how much). I'd still like a way to limit that but as i dont have a spare LM2596 i cant test that.
I've been 'tinkering' with the thing today and completing the coding and all 'seems' to work - with simulated solar input. Time to move off the test bed.
polymorph, thanks i shall have a look at DesignSpark PCB
Thanks for your help people the answers and questions were appropriated - an aid to thinking