OK I've determined that water is getting in in three places

A/ Under and round the roller shutter:
The old bottom seal was replaced yesterday but the seal to shutter joint leaked. Last night I pryed back the outer upper lip, put roof seal gunge under
it, re-seated the seal then put more gunge in the angle where they join, and am pretty sure that is now water tight.
The threshold 'kerb' that it comes down onto is uneven leaving a 1/2" gap in places - I'd intended to chop it out (it's soft sand and cement with added
PVA) and re-instate it in sharp sand and cement but I've changed my mind - I'll still hack it out, but I've ordered a commercial moulded rubber
threshold that incorporates a step as a dam and is bonded down with Everbuild Stixall that by all accounts seems amazing stuff and even can be
installed under water (It may have to!)
B/ Where the foundry corrugated sheet roof meets the bargeboard above the roller shutter.
We tried sealing this junction with 'eaves closer' foam inserts and roofing gunge - it improved things but wasn't a full cure. It is this junction that I tried
(and failed!) to improve today. The re-inforcing mesh arrived so I was up on the roof while the weather was good trying to install it. Sadly the mesh
hasn't got the 'give' to sink into the corrugations and still remain on the bargeboard - it just spans across. I had to pull off what I had done for a re-
think. It did however successfully let me seal two gaping bargeboard joints.
C/ This is what I think of as the 'original leak' as it was the first that I was aware of.
Water is running down the outermost corrugation of the main workshop roof and straight into the foundry. Theoretically water should never enter this
corrugation as it is covered by the upper face of the bargeboard, and the sheet to bargeboard has already been taped and sealed as has the moulded
ridge capping. This corrugation emerges into the foundry due to the history of the three buildings being built sequentially and the roof being (lets be
polite!) imperfectly designed. I think that this leak is going to prove the hardest to cure.