I agree.   Microsoft has “got you on-the-bones for” £10,000.   And, guess what:   it would cost you more than that (and risk much more than that...) to change it.

I agree that in the long term you want to move away from Microsoft Access, at the very least as the data store mechanism.   But it is absolutely imperative that you treat this as a formal project, with a formal project-plan, a timeline, a budget of its own, and a risk/alternatives analysis.   Some players very, very high up on the command-chain in the enterprise need to be brought into the picture, because it is within their realm of concern.   This conversion is probably something that needs to be done.   The conversion is going to cost more than £10,000.   The expenditure can be quantified, and probably, justified.

You might say to yourself, “but why do I have to do all that?   Umbuntu is “free,” and I know I could just cobble something together ... ... ...”   (your voice trails off...)

Abandon all hope, ye who enter here.

“Danger, Will Robinson!   Danger!!”

Incidentally, one way or the other you will have to do this, even for the (apparently inevitable) course of upgrading the Office installations!   You have a “mission critical” app here, or at least, one that is being used every day by 35 people.   The conversion process has to go “without a hiccup,” and that, also will require a formal project plan.   Do not assume that the software will run without modification in the new environment:   this is almost never the case with MS-Access applications.   You will need a testing environment, a beta-test cycle, a rollout plan, the whole #!.   By the time all the costs are accounted-for, heck, this might be a £20,000 project... and that’s just a cost of doing business in this world.   (If 35 people rely on this thing, I’m sure that cost-justification won’t be a serious problem even with some larger figure...)