Well, I don't know what to think!

Thanks for all your responses and advice.
Liz08 there were no interior photos of our house - all they had was an external photo which was at least 6 months old, and they claimed they'd seen interior photos, but they clearly weren't of our house from the description they gave us.
jenpax that is exactly what I think was happening here.
grandtante no, we've never uploaded photos of our house to social media, either interior or exterior shots, so the photos they claim to have seen must be from another similar house, or they were lying.
trisher yes, I've checked airbnb and it's not on there. I'm positive they had no opportunity to take photos of our home while we were talking to them - thankfully we were on our side path with the front door shut so they couldn't see inside at all.
Some of you have suggested it looked like a 'distraction burglary' and I agree, it was one of the first things I thought, but there was no opportunity for anyone to gain access to our house, and I checked on the obvious 'easy to grab' things immediately - nothing was missing.
I have spoken to Action Fraud, who have taken a full report and are escalating this to the National Fraud Intelligence Bureau who will investigate.

It could have been an attempt at distraction burglary, or straightforward burglary if you weren't in, but turned out to be unsuccessful.

Or just an excuse to stand around having a good look at your property.

Glad you're getting it checked out and upping your security.

It's scary, but these days there are countless ways of getting information & photos of people's properties, and the people themselves. You may have put a photo on social media, perhaps of yourselves or a family member with your home in the background. Have you tried a general internet search for your address and your surname to see what comes up.? But the most likely source is Google Street View or Google Maps, or a drone could have been used to photograph homes in a target area. As others have said report it to the police just so that they know of that activity in your area.

We have a very good system where I live (I'm sure others do as well) - we are on an email list with the local police, run by the PCSOs. We are informed of crimes in the area, reminded about safety precautions etc. They also inform us of odd occurrences such as this, saying 'we're not sure what this is about, but be aware'.
When that happens, you often hear of others, and whatever the 'scam' is becomes apparent.
Hope all remains quiet for you!

Happysg no, as I have said, I have never put any photos of our house on social media, and yes I have tried a general internet search for our address and nothing comes up other than the usual "house prices/houses for sale in (street name)". And no, our house does not appear on any of these Google results or any other searches I've done. And yes, I agree with you (I mentioned it upthread) that it could be Google street maps. It is, as you say, very easy to get photos of anyone's property these days - even without a camera! Unless, of course, you're lucky enough to live enough to live in a private road that doesn't appear on Google maps.
And yes, I did report it to the police on Sunday (the day it happened).
Nannarose that sounds very good - I'll have to see if there's something similar here. We do have a Facebook page for our area for people to report things, and the local police also have a presence on FB.

I know it will be a bit of a pain but do contact the police. It is probable that 'someone' has manipulated the Zoopla site and made a fake one.

Quite how a 'prospective buyer' would hope to get past you, I have no idea but there is something fishy...