This is Anna's first crack at caving, as opposed to simply wandering around the slightly more predictable, if no less dramatic show caves such as you'd find at nearby Wookey Hole. Having confided in me that she goes unusually quiet when she's not comfortable with risky situations - real or perceived - her tight-lipped demeanour speaks volumes as we pull on our boiler suits, helmets and lamps, and Chris delivers his pre-trip safety briefing.
But the nerves diffuse somewhat as we stride past the impressive calcite formations in Gough's show cave, attracting admiring (or maybe pitying) looks from the "tourists". Then we branch off into a small passage to begin our mixture of scrambling, climbing, stooping, crawling and swearing through the Mendip limestone.
Having visited a number of caves sporting chambers with names like the "Eiffel Tower" (because you look up and get an eyeful), I immediately assume that Mushroom Chamber had earned the name on account of its size. But no, apparently the cave guides in the 1950s actually thought it would be a good wheeze to try growing mushrooms. The experiment was a dismal failure when the mushrooms died - all that remains now are a few scattered remnants of peat on the floor.
The equally imaginatively named Boulder Chamber is noted for its short moment of glory in the 1960s, with David Laferty's world-record 130-day stay underground. "He had no idea of the time," Chris tells me. "He became quite bored and depressed, and he slept a lot - 30 hours at a time." The fame was short-lived, however, as a Frenchman broke his record just a month later. These days, the most striking accessory in Boulder Chamber is a bright yellow BT telephone - surely one of the least accessible and doubtless least vandalised telephones anywhere.
"So how long do you think it'll be before your eyes get used to the dark and you'll be able to see something?" asks Chris when we all switch off our headlamps. An embarrassed silence follows as nobody dares speak in case they get it wrong. The answer, of course, is never. This isn't just gloom, it's total darkness.
As caves go, Cheddar is pretty dry. Some of my previous caving trips have had me paddling in streams, even wading neck deep in icy water. Of course, the real enthusiasts (or head cases, depending on your point of view) go for cave diving, exploring flooded tunnels and chambers with the aid of sub-aqua gear. But for now, I'm quite satisfied to follow Anna's example by joining the line through one particularly damp passage behind the other members of the party, so the really puddley bits are mopped up by the boiler suits in front.
Just when you think you're getting into the swing of things, along comes the final surprise, "April Fool's Squeeze", a small passage which has to be taken at full stretch, with head down, and feet up. It helps to be thin - actually, I reckon it helps to be double jointed as well - but I gather this particular squeeze does take people with chest measurements up to 48in.
The beauty of the Adventure Caving Trip provided by Cheddar Caves is that it's one of the few places in the country where you can just book up and go caving. In the normal run of things, you'd have to seek out your local caving club. "We get all sorts caving with us," explains Chris. "We take children as young as 12, and we've had people well into their seventies." You need to be relatively fit to do the trip, but even so, you'll almost certainly find one or two muscles you'd forgotten about reminding you of their presence afterwards.
Martin Ridnell came here from Northampton with his daughter Hannah, 13, so she could pick up a badge from the Guides. She thoroughly enjoyed the experience. "Great value for money," adds dad. Jonathan Clark and Gill Orme, recently moved to nearby Glastonbury, booked their trip into the cave as part of getting to know the area - what most might regard as a somewhat extreme piece of reconnaissance. Gill had been visibly quaking at the start of the trip, but once we got going, she was scurrying about, investigating different tunnels each time the rest of us stopped for a breather. In fact, she and Jonathan enjoyed the experience so much they decided to join their local caving club.
Even wannabe-Bond-girl Anna, once she too overcame her initial flutters, had as much fun as the rest of us, admitting afterwards that she "felt like a kid in an adventure playground".
CAVING IN THE MENDIPS
Cheddar Showcaves' Adventure Caving Trip lasts one hour, and costs pounds 10 for adults, pounds 7.50 for children. Boiler suits are provided, along with rubber boots, although you can choose to wear your own walking boots. A helmet and lamp are supplied, along with a belay belt and karabiner for safety on a couple of sections where a slip might result in a fall. Cheddar Showcaves (tel: 01934 742343).
WHERE TO STAY
Clive Tully stayed as a guest of The Crown at Wells, Somerset (tel: 01749 673457).
For more information and a free guide to the area and available accommodation, write to the Mendip District Council, Cannards Grave Road, Shepton Mallet, Somerset, BA4 5BT (tel: 01749 343027). Or email them at: firstname.lastname@example.org.