Bought this glasscast 3 for a project about 8 months ago and worked very well.
Mixed up a litre today by weight 100/50 as per instructions for 10 minutes. Steam started coming off and went totally solid a couple of minutes later....... really not happy !!!!!

Who told you to mix it for 10 minutes? Epoxies don't need to be mixed for 10 minutes, 2-3 minutes should be plenty. 

Glasscast3 is a low volume coating resin. A litre of resin in the style of jug you have used there is a substantial mass, so the Glasscast 3 will go off very quickly. It is supposed to be mixed for 2-3 minutes and then poured, so doesn't sit in a mass for too long.

Mixed roughly for 10 minutes in total. Swapping jugs half way thru. This is how I’ve done it before with no issues and I’ve mixed glasscast 50 the same way without ever having problems

Yes, glasscast 50 is made for thicker pours and therefore less reactive and doesn't build up that much heat when mixed unnecessarily long in large batches. You can't do that wich glasscast 3. Unless you had a huge surface area to cover, the pour would have been to thick for glasscast 3 anyways.

Eh???
I mixed enough for 80cm x 26cm and a 3mm depth.
I know glasscast 3 is only a thin surface resin and I’ve used it before successfully.
It was going onto a glasscast 50 pour which had 2 pours to reach a depth of 40mm.

80x26x0,3cm=0,62L. And 3mm is the Upper end. But that is not my point. You simply can not let epoxy stand around (or mix) in a compact 1L container. The reaction will build up heat, which accelerates the reaction, wich accelerates the heat production and so on. This can only be done with extra low reactivity resins like gc50.

I see and I get what you’re saying however I followed the instructions correctly.
There should be limitations written on the instructions as to what not to do then if this is the case.
As far as I’m concerned, I followed the mixing instructions correctly but I appreciate what you’re saying so thank you 👍

No, it is clearly advised to mix for 2x3min.