Do you know what anti-condensation membrane you have beneath the slates, and its condition?

Hint. When refitting slates, use black stainless slate-hooks. That enables you to regain access at a later date if you need to.
I can provide more info if you need.

If those wasps hadn't been exiting behind the water cylinder, how would you have known they were inside the wall?

Because we can see them entering via the slate roof tiles above the utility room. It's the first year they nested there, and we'd get 8-12 coming down the wall and entering the utility room... we'd open the windows and let them out. But there was a lot more entering and exiting via the roof. Maybe it makes more sense to try and seal the roof and prevent them coming in to start with.

Unlike bees, they don't limit the size of their nest.
How big do you think it would grow inside the wall cavity before you actually began to hear them?

On the plus side, wasp nests are exothermic.
So, if you wanted to, you could block the bottom of the wall, let them build as much as they want, and increase your EPC 😲 

There will be others wanting to know what works... and, just as important, what doesn't!

Predators include: Praying mantis, geckos, hedgehogs and badgers.
Useful to know, but not a great help if you want to prevent a nest inside the wall.

Aversions: wasps don't like certain aromas, including citronella, peppermint and spearmint.
Those would seem to offer a better approach.
You could either stuff mint leaves into the crack at the bottom of the wall,
or alternatively put drops of citronella oil into any crevice through which you think they are entering the house.

As a general deterrent, I can't see any problem with hanging bunches of mint stems in an attic.
They'll slowly dry during the summer, and leave the loft-space smelling great!

The Illbruck foam differs from generic PU (polyurethane) for two reasons:

• it retains a specified elasticity after curing
• it has greater adhesive properties

Whilst I can think of situations where those attributes would be especially useful, I wouldn't have considered them of significance for the wall-floor gap mentioned by @editor
I asked the question because I'd expected you to raise those issues.

I've checked prices today, and the brands of PU foam which I'd generally use are currently around £55 (for 12).
The Illbruck FM330 is between £112-£120 ish.

So I'd only be buying it if I was particularly after those attributes.

Now if only we could find a foam which incorporates insecticide properties, then @editor would be happy 🤨 

the foam itself is a specific air tightness foam.  Standard foam could apply but I think for the couple of £ per tin it’s better.  Also the tape is seal sealing and very robust.  Try eBay as you can find rolls reduced.  

Tescon Vana tape is also an unusual product to find being recommended here.
It's intended for airtight seals between insulation, moisture repellent or low-humidity sheeting during construction.

Both products are expensive due to particular mechanical features identified in their specifications.

or if you can get your hand in buy some Tescon Vana tape and seal the whole rooms perimeter.