Thanks all - I have seen all of those web pages - interesting but not useful in my current situation.
Unfortunately, I am not about to break the coal bunker off my loco (when I glue something it stays glued.
Here is the beastie in question. My second attempt at a kit build - Why no one is making a ready to run Tank is beyond me (Shrike models? I think you have one planned for 2016....?)http://i44.photobucket.com/albums/f28/Carpo5/Model%20Railway/Z20%20class%202013_zpskmnfixsh.jpg
I think a ZEN decoder with stay alive may be the solution - I don't need a lot of capacitence - just to get past the little stutters and shudders.
It is a pity DCC Concepts don't publish more about their products - some of their info is great - but fundamental info about a stay alive such as capacitence and dimensions should not be too much to ask.
Having said that, I sent an email to DCC concepts and they replied:
"Our smallest stay-alive unit is 21/9/5.5mm"
I will get one of these and add a post here with my thoughts.
I considered making one but stories of popping capacitors (I have seen them go - not pretty) scare me off.
I am a mechanic - not an electrician!