Thank you all for the input and suggestions.
In the cold light of day I have decided I can't afford the Spa wheelset, lovely as it is. Also as my front wheel is already upgraded and not giving any trouble I don't relay need to change that.
My options seem to be...
doog's suggestion of Parkers sputnik with a tiagra 4500 hub for £53.
Or my local shop building me a Mavic 36 spoke 319 with an STX hub and DB spokes for £45.
I like to buy local if I can, how does the mavic/STX compare with the Sputnik/105?
My other concern is that my current 7spd cogs will fit as I don't want to add another £20 - £30 to the bill. Can I confirm that my current one is indeed a splined fit without actually dismantling it. It's marked - MF - ZH SIS 37-7.
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Willem
Your suggested 15kg sounds great and might well be easily achieved by some people in certain circumstances, credit cards are indeed very light.

Unfortunately in my own circumstances I have spent both money on equipment and time with scales to achieve the weights I carry now. My best would be perhaps 20kg but this would be very restrictive and inadequate for my usual trips. Touring with a campsite or even a BB waiting at the end of the day is a totally different proposition to my own quests for a self-sufficient escape to the wilds.
I only take one change of clothes and these are all lightweight. My camping kit is as light as it could be while remaining sufficiently durable and my liquid intake is dictated by both health needs and habit.
bikepacker
Thank you for your generous offer.
I think the consensus is though that my current hub would be a waste of decent components, not to mention your time and skill.