I'm wondering what "computer" they used.

If they didn't use "Peugeot Planet" diagnostics some very odd results can confuse the issue.

And Peugeots do suffer from electrical faults that could fill pages here.

As your garage are probably well aware.

So what I'd do is get a "Global Scan" done using Peuget Planet,

so you may have to get this done at the dealer if your garage doesn't have P.P.

What a global scan is basically the computer scans EVERY system on the car for faults then reports.

So I'd ask your garage if they used P.P and if not find someone who has Peugeot Planet and get them to scan it.

I'm not suggeting your garage is at fault.

It could be somethig as simple as a fault in the seat belt system that happend say when the tech adjusted the seat to bring the car into the shop.

Scan it with P.P.

The ABS and engine controllers communicate with each other, and engine power output will be reduced when the ESP system is activated. If the valve timing was not spot on correct then it's possible this fault could occur. Failing that the most likely explanation is that the garage have inadvertently disconnected something or forgotten to reconnect. It wouldn't necessarily have to be something to do with the braking system. It's just as likely to be something on the engine or an earth. Or even a communication data wire.

There cannot be a long term future for independent garages now as technology on cars advances. Gone are the days where motor mechanics found their way day to day. These days you have to have the correct knowledge, training and factory support. Only main dealers will ever get that.