UKH

Got a full set of hexes from an old friend of mine, the metals in really good nick but the slings are fluffy as owt, whats my best way to re sling them? Or use the cord off needlesports with a triple fishermans?

I did 7mm nylon with double fishermans. 5mm dyneema with a triple would be lighter but I'm happy with the result. I use them mainly for winter and it feels like they're less likely to get cut with the fatter cord.

Needlesports do a size chart somewhere - 

https://www.needlesports.com/catalogue/results.aspx?search=Hexes

I'd be tempted to use sewn 60cm 8 or 10mm slings pushed through double. Clip both ends for short, larksfoot and pull the sling through to clip a single loop for long.

Cord is traditional and works fine or 18mm tubular tape.

jk

5.5mm dyneema will do old fashioned camp Hex 2 (with a slight fight threading), Hex 3 and Hex 4 but will be a bit loose on a Hex 5.

One thing to consider is the length of the cord. If you make them longer than Needlesports suggest you can use them without needing an extender. Which is excellent for scrambling/some alpine routes e.g. Cuillin ridge Hexes 1-5/6 with a krab each and some slings; light weight and job done.

Downside to a longer length is that in winter they may hang a bit low, and risk entanglement in crampons when high stepping, hence people racking higher on a bandoleer/sling or doubling up to shorten.  In summer the smaller sizes can entangle with larger cam lobes, so thought to racking required.

Sling slings?  (Not cord?)

If they're Torque Nuts, DMM offer a re-slinging service for them:
https://dmmclimbing.com/About/Returns-Servicing-Repairs

If they're not, but you'd really like new slings on them anyway, don't mind spending a few quid and aren't in a hurry, you could send them off on on a Californian holiday:
https://www.mtntools.com/cat/rclimb/cams/mt_camresling.htm

If you’re going with cord, extra style points for knot inside the (larger) hexes

Downside is it doesn't let you reposition the cord to even wear at the 'corners'.

On a dark wet final pitch (or  a dry day) is it feasible that you have lent some of your rack to your mate and that they clip one end  when not larks footed ?

I have some nuts threaded in this way and it worries me—- sometimes 

Standardisation, or 'one less thing to think about' is always a good thing. I'd probably change them just to have one less thing to think about.

Edit: If the worst happened you wouldn't want it as a regret. (I'm always a cheery sort)