Firstly, a great many thanks for all the replies. I will sit down and deal with them when I get a mo. For now, here's a picture of the loco in question. It is 30 years since its last overhaul and is due a big boiler tear-down and inspection not to mention anything else that we find along the way...
Quite a few suggestions of a spreadsheet such as Excel. Sure enough they are very flexible and just about anything is possible but at the same time they are fragile and hard to maintain. I might use one for a personal project but have my doubts about using one for a small team many of whom are not computer savvy. Since so many people have suggested I may well have a play and see where it takes me.
I had in mind something more like <<this>> which I use for my family history project. You buy (and thus own) the code and the host it as a website on any commercial server. You also own all your data which is not true of many 'free' providers. Interestingly this has many of the features on my wishlist but it would not be easy enough to adapt it for my use. Hosting it on the cloud covers all the security and backups.
My users are mostly elderly so the system must be pretty simple to use.
Heavily restricting those who are allowed to make changes was how BT stopped their various Excel spreadsheets being wiped , adjusted messed up . Only very competent people who had had a few months training and passed the Excel exams were allowed to change things and even then it was tiered access.
That's very much like a BOM type structure (bill of materials). They both have a multi level nested structure, so your inventory consists of multiple assemblies, sub-assemblies, components, etc. with each level 'belonging to' the previous. This allows records to be created and updated only at the level that you're dealing with.
Excel is very capable, and I have created some complex, multi user things with it, however the fundamental issue with it for problems like this is that it has a flat data structure. e.g. you move the RHS cylinder assembly to a storage location so you need to update the records for the RHS piston, RHS piston rod, gland plate, front cover, rear cover, valve, valve rod, valve chest cover, cylinder studs, front cover nuts nuts, front drain valve, rear drain valve, etc. etc.
In a nested 'BOM' type structure, each entry is related to its predecessor - you only need to create / update a record for the RHS cylinder assembly and everything under it will follow. At any time you can add more sub-assemblies to it, and each of those can also have sub-assemblies until you are down to component level (if you want / need to go that far).
It is tempting to try and emulate this in Excel by adding multiple sheets, but each time you do this, you multiply the complexity, so one or two layers extra might be workable, but it rapidly becomes unmanageable like the puzzle about how many grains of corn it takes to cover a chess board if you put 1 in the first square, 2 in the second, 4 in the third, etc. (Been there, done that.)