Ive done all the research, set up a factory to produce all the components, and put several of the 6 final titan components in the assembly bunker 'titan body ingredients' chest.
However I cant figure out how to build a titan. I have activated the assembly bunker. I clicked the green 'start assembly preperation' button, and get a box 'status - titan class is not set' and 7 empty boxes. Click a box, another popup comes up with 'set the filter' and it says 'no recipe avaliable'. I cannot get past this point.

First of all, if you see "no recipe available", it means that you haven't unlocked any recipes of titans or weapons with corresponding researches. First of them are "Class 1: Scout Titan" and "Titan weaponry grade Minimalis".

Note that these researches are conducted using special WH40k Lab and Titans science packs – which you can produce (in case you have an advanced factory) or reverse-engineer found fallen titans.

After you research and specify the recipes, you will need to put weapon details and ammo into specific storages, different from body parts storage. Follow tips from the status line and lamp indicators.

If you still have any issues, provide more information about what researches you finished, what unlocked recipes do you have, and what other mods do you have (overhaul mods often affect technologies and recipes of other mods).

I eventually figured this out... it just needed the mod updating as I had updated factorio itself....