As a quick hopeful fix, I'm going to experiment again.

Hopefully, it's just the fork oil degrading. As I said, it's a recent thing.

I'm going to try some heavier oil, since I've now discovered the original oil was 10W, I bought a litre of 15W and I'll give that a go before I try the Ohlins!

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Bike: 2013 XT660Z, TKC70 tyres

Mods:
Barkbusters, Scottoiler, DIY Tail Mod, DNA Stage 3, Power Commander V & O2 Eliminator, Exan X-Black Stainless exhaust, Tool tube, Ohlins linear front fork springs, Nitron rear shock, JT front and Supersprox rear sprockets, D.I.D. VX 520 chain, Adventure Spec bashplate, DIY USB Outlet, DIY chain guide, Kev's Throttle mod

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