Considering a new frame which doesn’t have any front mech provision.

Currently have 2013-era SLX 2×10, with (I think) a long cage rear mech and a Deore HG62 11-36 cassette.

Is it still worth going down the route of conversion sprockets (and I think I’d need a new cassette anyway), a new chainring, etc or is that all a bit silly now with XT and GX at £300ish for a whole new groupset?

Superstar have narrow-wide rings and expander cogs at 25% off – so you could get those and a new XT cassette for about £75.

I think the XT 11sp cassette costs almost that much alone.

A triple/double&bash crank would be a better starting point than a “proper” 2×10 crank like the M675 SLX – the ring offsets are different with a “proper” 2×10 crank so you may have to live with a less than ideal chainline if you decide to go 1x on your existing cranks

Yes a triple is a better place to start, but as you aren’t starting from there… Its not a big deal to sort the chainline. Swap any BB spacers over to the other side and/or washers between the ring and the crank work just fine too (if you have long enough chain ring bolts). All you need is ruler and some lateral thinking rather than new cranks.

Steal it all.

Side effects may include prison and bumming.

Sounds to me like you need an excuse for new kit…it’s ‘only’ £300 after all. 😉

Otherwise look at the Groupsets on CRC and get one of their 10 speed set ups and use current cranks. Job. Done.

You don’t really need a gert big sprocket out back. Just get a 30 tooth front and 11:36 cassette and you’ll be reet.

If you need a new cassette, and you probably want a clutch mech and an expander – then the XT 11 speed setup and a narrow wide up front would be a better move overall, or go the whole hog and get the group, and sell your old mech, shifter and cranks.

Expanders work well, but they’re a bodge one way or the other, and XT 11 is proper. You’ll have spent most of the money to end up one gear less.