So, apologies if this comes across as a rant.

We had a nice Mercedes E220 2014 plate on hire from Heathrow from Friday - Sunday. All was going well until Sunday 11am, when we made a simple mistake. My husband put his jacket (with car keys in the pocket) in the boot and shut the boot. Unfortunately, this locked the car with the only keys in the boot. Naturally, we were annoyed but glad our flight wasn't until 15:30 and we had allowed plenty of time to get to the airport.

As almost all our belongings were in the car and we had no access, we began to research online who we could phone (thank god I still had my handbag).

First thing we noted - it is impossible to phone the roadside assist number from a non UK phone, as it is a free phone number.

Number 2 - the 24hr Heathrow airport customer service number only goes through to an answer machine.

So, we phoned the reservations department, explained our situation and were put through to the AA, who attended within the hour. Unfortunately, they were unable to do anything (although the guy was great). In the end, we agreed to pay £300 for a local locksmith to come and open the boot - the AA's key team don't work on weekends.

Unfortunately, the locksmith was unable to open the boot, although he was at least able to make a key that could open the car. But, no access to boot from the main area of the car.

So, back to Hertz customer service for the 10th? time. Why so many times? Well, in order for the AA to be allowed to do anything, they had to have permission from Hertz - this included trying to call out the non-working key team! In the end we had permission to have the car towed to Heathrow (not easy from an underground car park with a rear-drive automatic parked backwards), but as our passports were in the boot, still a major problem. We tried to sort out an extra day's hire so we wouldn't receive a huge charge for late return, but were told to phone the branch (who didn't answer their phones). Twice we were advised to email in for help despite being stuck in an underground car park with no belongings but a local locksmith having a key for the vehicle - thus we were unable to leave it.

Eventually, I had the sense to call Mercedes in Germany. After a stumbling conversation (apologies if you are reading this - usually my German is much better!), we got put through to the UK team, who sent a Mercedes Assist van out. After around 15 minutes, the boot was open free of charge.

At no point had Hertz or the AA suggested phoning Mercedes. All I can suggest is that if you have a similar problem, don't bother with the official procedure. Simply phone the manufacturer and try to get it sorted.

We are now out of pocket for new flights, a night in the hotel (we missed the last flight of the day), a drop-off fee (couldn't drive after not eating for 9 hours and feeling stressed) and an unplanned day off work. I doubt insurance or Hertz will pay up, but hopefully our story can prevent other FTers having the same problem. Oh, and Mercedes (if for any chance you see this), do you think there is a chance you could design a mechanism so it is impossible to lock keys in the boot?