z1ppy, was all the kit new at the same time? ie. was the bike bought as a whole, or already used kit put on it?
If it’s all in the same nick (ie. was all bought new together), and it’s in good condition, looking at its age and relative desirability (being a popular bike), I’d say between 50 and 60% of it’s brand spanking new ticket price (though perhaps discount the RRP on most things, more the price you can buy them for on the net).
I’m guessing the bike was about £2500 new, so your first guesstimate of about 1300-1400 wouldn’t be far off I’d say.
Of course if parts are knackered and going to need replacing soon, then lower the cost accordingly.