Hi,
When I push the starter button, it takes progressively longer before the starter motor turns, up to 35-40s now.
When it does spin, there seems to be plenty juice, any clues where I start looking?

First, confirm the battery is good.  Then, after first double checking the bike is in neutral, with a known good battery and heavy cables, positive to the lug on the starter (under the rubber boot) then negative to starter case you have bypassed the starter circuit so the bike should now crank nicely if the starter motor is good.  If not you have high resistance in the circuit, a bad solenoid or a bad ground. 

This should help you

Starter Circuit Voltage Drop

 I've noticed that if the motor doesn't start on the first attempt, I don't continue to stab the start button because I've had simular results as you.    Turning the key off and on to begin the starting sequence again and holding the start button until the beast fully fires up or continue the on-off method, seems to work for me.

Thanks guys, useful advice. When I get a chance, I'll dig out the old multi-meter