Hi !
You mention you have spent around £550 on the bike since you got it !
Unsure if all of the things you've changed were actually worn out etc or you have done this for your own piece of mind.
Local guy to us just sold one of these. Same year, model with pants, helmet, boots and bike rack for £1100. IMHO I'd say between £800 and £1100. All of the new bits you've added will surely make it an attractive buy for someone as it's all stuff they should not need to shell out for.