Thanks all for your comments so far.


This is the reason for my question......is it likely to be the solenoid itself?
Is there a way to check it before cutting into my wiring loom?

 

There is no need to cut into your wiring at all.

Simply remove stock female spade connector from connector block and replace with a regular one.

You will need a very small screw driver to press down the tab to get out the oem connector and maybe a pair of small needle nose pliers to insert new connector through the block (this is what I did but it has been a while).

Don't forget to wrap up oem connector with electrical tape!!!!!!

 

You can also remove the cover from the end of the solenoid and take a look at the contacts.

If they are looking tired, you can get all the parts at a starter repair shop.

 

Deleted double post...

 

All good advise . Check all connector blocks and wire connectors etc.
A wet paste of arm and hammer and water will remove the elec. corrosion .
Flush off the paste and add a bit of white grease to the connectors to prevent future corrosion., this is standard operating procedure with old wiring / connectors.
I suspect your problem is a dirty start switch , disconnect battery , use WD40 or your choice of cleaner and with the nozzel provided , shoot a spray directly into all switches while activating the switches . When what drains out is clear , that switch is clean. Do all.
If you have access to comp. air blow out the switches , if no access to comp. air let bike sit for a night or blow out the switches with a straw and lung power.
You will be surprised how much crap is flushed out of 20 year old bar switches.