Many thanks for all your suggestions and advice.
To date I have replaced the wiring between the starter solenoid and starter with new cable also new earth leads between battery and frame and frame to engine.
It is currently running the old cabling between the starter solenoid, two way connector and battery which I will replace shortly.
I have rebuilt the starter with new brushes.
I have also substituted the starter solenoid from my 600 Monster on the 900ss.
When pressing the starter the solenoid appears to work (and on the 600 the starter spins the engine as though it has no plugs fitted.)
The comment about the two pin wire into the starter solenoid is something I will look into further as I suspect this has a potentially poor connection however it has always tried to turn over so I feel it may not be the main cause just a potential problem for the future!
When I try to start the 900 and it fails to fire it tries to crank but its as though it has too much compression and just can't get it over tdc.
On other occasions it fires on the first crank.
Frustrating and worryingly inconsistent!!
The battery I am using, or rather batteries as I keep swapping them over, both seem to be in good order. One of them is a yuasa and I had that charged and checked at work as good.
I think I have two problems here, one of which is typical of wiring on older vehicles, corrosion and poor connections, especially where the previous owner may have meddled with it. Not uncommon with a 14 year old Italian motorcycle!
The other is the battery going flat if unused but left connected on the bike. Means there is a drain somewhere that I have yet to find. Something that doesn't happen on the Monster fortunately (touch wood!)
More than twenty years ago I used to earn my living on Italian motorcycles, mainly Guzzis and Laverdas, so I should have known better than to buy an old Italian motorcycle!!.
Still once motorcycling gets in your blood there is no turning back and I have always had a soft spot for Italian bikes
Still, when it goes its worth it.
Will update you with my progress.