UKC

I don't want to reinvent (or build from scratch) the wheel. I'm looking for some kind of wall mounted external locker/cupboard. I want to hand bikes 2 high and side by side (4 in total) on blank section of wall to keep bikes closer to the house and free up room in my outbuilding (Victorian pig sty).

Anyone recommend anything that will do the job. Maybe two side by side, I've found the hangers for the bikes). Ground slopes away from house so floor mounted won't work.

This sounds like a rather unusual application, something that you will likely need to build bespoke. Maybe just build a small shed against the wall? 

My only real advice is to not skimp on locks and install a solid anchor into the wall.

I was hoping someone could recommend a prefab unit and I was going to mount it. If not, yes I've got to build small sheds.

I was going to use secure attachments to the wall and bike to hanger, the shed bit itself needn't be.

What you are describing is what Asgard manufacture:

https://www.asgardsss.co.uk/3-to-4-bike-storage