Paul, seems a nice amount to lose and I'd bet that the eventual replacement guard and tail light setup will be about 2Kg so you're a winner.
However I think you'll churn in mud with those tyres, better fit some TKC80's, eh?
I recently (for a Christmas trip) fitted a home made timber rack/platform for putting on my Ortleib bag and tent on top of the bag. Made it out of exterior 5 ply timber and cost a total of $6.00 Australian or something like £1 in your, money. Works a treat and I'm now going to make another with the leftover timber for when I'm travelling solo and want to run the Ortleib bag lengthways, behind myself.
Mick.