So, I bought some of aforementioned gunge, and have tried two of the packs. Both came out really lumpy. I don't have time to get a mixer, short of getting a cheap electric whisk from a shop (is it worth doing that?).

So, how do I avoid lumps? One solution I tried was start the water then the powder which produced less lumps than the first bucket, but still has a lot of lumps.

I've had good results with the powder first at the bottom of the bucket, levelled off so it is even distributed. Then I have added in one pour the 10-12 litres water that I've already measured in another bucket all at once, making sure also that it is the right temperature - warm but not too hot. I then stir with my arm in a circular motion until you can feel it start feeling it thicken. Messy Supplies has good instruction videos. I've mixed the MS gunge, and also Natrosol in this way and it has turned out fine.

Yeah, I've had no issue with the natrosol gunge, but I'll take your advice, and try that. Thank you.

I add powder to the bucket first, then fill using a shower head quickly swishing my hand through the powder almost as soon as the water hits. Seems to stop most of the lumps.

For guaranteed no lumps, use a sieve to gently add powder to water while constantly mixing.