Seems to be affected by ambient temperature, probably only noticed it since the start of winter.
In the mornings the 850 takes a while to get going with 2 or 3 goes at starting, then it splutters out if anything more than a tiny bit of throttle or load is applied.
Choke is on to start.
Takes a bit of revving to warm up a bit and then it mostly goes alright. Will intermittently splutter/misfire randomly for up to 2 minutes after starting. Runs fine after that.
Carb is a single SU, using really really light oil in the dashpot (feels similar to WD40).
Could this be a trouble or is it more likely to be ignition?

I have always found WD40 too watery, I use 10w air tool / compressor oil.
Make sure the fast idle is set correctly but a good check over both fuel and ignition will help.

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Cheers Dave.

Yes you need something to slow the rise of the piston a bit to prevent lean-out.

Auto trans fluid has been my dashpot choice for decades.

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Cheers,
Rod.
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'63 Morris 850 (Old Secondhand CAR)

Thank you both, I will try air tool oil and report back tomorrow.
What do you mean by 'fast idle' - is that idle under choke?

When you pull the choke all the way up, it should also open the throttle. There is a screw to adjust the fast idle setting

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Thanks, yes it does increase the throttle quite a lot.
Air compressor oil in the dashpot (boy these are easy to overfill!) has made all the difference to starting and driving while warming up - thanks so much everyone!