how are your troubleshooting skills with an electrical schematic?
I've attached a wiring diagram for the non-ABS model, if yours is ABS2, then there might be some differences, but probably nothing that would apply to the issue you're having.
Since you have a new starter relay, let's assume that its good. There are two ways the starter relay energizer circuit could go bad, either no voltage on the yellow/red wire, or no return to ground on the green path.
The yellow/red wire comes right off the starter switch, so put a meter on that wire at the starter relay, press the starter button, and observe if you get 12v or not. If that checks out, on to the ground path. If it doesn't, then there are a couple other things in the path to check. When you turn the ignition switch on, it energizes the bank angle sensor relay and then the kill switch is also in the path, so make sure you have the kill switch in the "run" position.
The ground path has two possible ways to travel. If the bike is in neutral, then it follows the path through the clutch diode and down through the neutral switch. If that doesn't work, make sure the sidestand is up and the clutch is pulled in and try again. That takes the other possible path with the clutch switch and sidestand switch, see the schematic for details. If it doesn't start in neutral, but does with the second method, then your neutral switch or the wiring to it is bad. Or, given that you're in the UK, maybe the switch itself is badly corroded and no longer making a good ground.
I've attached a complete wiring diagram schematic, plus a starter system specific schematic along with a picture that shows where the various components are located on the bike. Good luck and report back with your findings.
