My 2015 Focus 1.0 Ecoboost went in for a service and mot yesterday, and was delivered back to my house. I first drove it this morning and pulled over after about 100m due to a very noticeable knocking noise coming from the engine. Upon opening the bonnet I noticed that the oil filler cap was nowhere to be seen. The car (should) have been driven no more than 10 miles like this.

I wouldn't have thought that driving 10 miles with the oil filler cap off would cause any damage at all, but the engine sounds awful and my very limited mechanical knowledge comes from much older motors so is probably irrelevant. Could it be that the engine is just losing oxygen because the cap is still off and that is causing the noise? Or is it possible that permanent damage has been done?

Have you checked there's plenty of oil in it?  I can't imagine they'd forget to put the oil back in but you never know...  

The cap itself being off won't cause a mechanical issue though you will hear a blowing noise from it.

The last time I drove one without an oil cap the oil spread itself all around the engine bay in a very obvious way. Was the oil pressure light on during the journey?

As Tom says, have you checked the oil level, if the cap was off they might've forgotten to fill it up 😨 I take it you've called the garage that did the work, what have they said?

Thanks for the replies. I haven't checked the oil level yet, I just jumped in my other half's car to get to work - I'll be checking the oil first thing in the morning.

The garage offered to come out to have a look at it, but I told them I'm getting someone else to have a look first then I'll let them know. I'd rather get a third party opinion before I give them the chance to hide or deny whatever they may have done to it. The whole thing was booked via the AA (but with an independent garage), so I've got them coming out in the morning, if one of their own guys says X then they can't try to pretend it's actually Y.

No oil lost, filler cap back on, engine all sounds good again. The filler cap was on the heat shield under the engine, had to jack it up to find it and get it out. Apparently the caps pop off if you don't push them down properly, although it seemed pretty simple last time I topped up the oil.

The owner of the garage came out with a spare filler cap just as the AA guy left and was very apologetic. Mistakes happen, no harm done, and he went out of his way on a Saturday morning to try to fix it. 

That's good then. Someone going out of the way to sort something out goes a long way with me, accidents happen to us all it's how we deal with them that matters.