It was 5.45am, and I found myself plunging into the dark and velvety waters of the lake. Above our tent, the silhouette of Old Wardour Castle was backlit by dawn as moonlight still played on the water.

My legs sank in silt up to my knees, Zam was nibbled by a carp and I made a fairly swift but undignified exit. Still, for someone not used to swimming in anything below 25°, I’d come a long way. It felt marvellous.

Earlier attempts to be a free spirit have met with various levels of disaster. I once suggested to my friend Rebecca, on an earlymorning walk in Norfolk, that we strip off and dive into the calm and glistening sea, which seemed to be inviting us in. I can’t think of a more unlikely pair to shed their clothes in broad daylight, but the beach was deserted and we felt uncharacteristically brave.

Minutes later, two pale ladies were jogging towards the water, which turned out to be less than an inch deep. On and on we splashed, feeling increasingly exposed, until, eventually, we just tried to lie down. The water didn’t cover us and it was only on turning back to shore that we realised a sizeable group of spectators had gathered near the piles of clothes that we’d unwittingly discarded by the 18th hole of the golf course. Some time later, I was asked by a friend to pose for the cover of his first novel, a request that, I admit, piqued my vanity, so I said yes before I realised what I’d let myself in for.

I had to lie on a concrete slab, at night, in the filthy waters of the Thames as litter lapped over me somewhere below the OXO Tower on London’s South Bank. For hours. Then, I had to wade in up to my armpits, wearing a diaphanous nightie. When I asked George why he’d proposed me for this weird experience, he explained that he needed someone tall and bony. He didn’t want my face in the picture and the resulting cover is of an anonymous corpse. I’m not sure how many copies of the book were sold, but I spent many months wondering if I’d given myself bilharzia.

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Yet I’ve become increasingly drawn to leaping into icy waters. When I last stayed with my friend Catherine in London, I suggested that I join her for her daily swim at Hampstead Heath. She gently pointed out that my heart would stop and I would certainly die. She was right. Temperatures in the pond drop to 2° at times and machines are used to break up the ice.

She swims 40m (no wetsuit, of course) and then swathes herself in wool and tweed, only warming up by lunchtime. She glows with more than satisfaction. My brother-in-law Barnaby enjoys a solstice dip each December. He builds a fire, and encourages us to leap into the lake. Sometimes, my daughter Anna plays a fanfare on the trumpet. The rule is that you must be fully immersed before you can swig from the whisky bottle, which is then handed round, each swig being spat onto the fire to an accompanying toast.

My children, who have been doing this since the age of five, adore it. I’m still a beginner in the coldwater brigade, but as spring rolls into summer, I’m much looking forward to ‘ottering’ down the shallow waters of the Itchen- another enthusiasm of Barnaby’s.

This inelegant manoeuvre involves pushing yourself forward on your arms while your legs-and often your stomach-brush over skin-breaking flint stones, leaving your head level with pond skaters and dragonflies. Sometimes, you enter a flow where the water carries you over the ranunculus and past the flag irises with little effort needed from your arms.

We’re discussing ambitious plans to set off from here, skim past the Worthys, skirt round Winchester, drift by Southampton Airport and emerge in the medieval salmon pond at Woodmill Pool, the last non-tidal waters of the Itchen. There is one thing that worries me, however. I’ve learnt to love cold fresh water, but I’m still terrified of swans.

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