It's raining and a soft blanket of fog is hanging over the Surrey hills as I turn off the A30 towards Grayshott Village, en route to the famous spa.
It's taken quite some getting here. Not just the bumper-to-bumper drive through the lunchtime London traffic, but the vast amount of planning necessary to enable me to take a whole six days off. From life.
A complex network of childcare, food deliveries, neighbours and playmates has been put in place to get one husband and two small children through to next Wednesday afternoon, when I hope to return lighter, brighter and altogether more human.
The programme I've signed up for is the Grayshott Health Regime, devised in collaboration with Stephanie Moore, a clinical nutritionist who works closely with the spa. The aim is to restore optimum gut health, and to foster a healthy attitude to food and eating based on sound clinical principles rather than short term fixes, fads or deprivation.
It's been something I've wanted to do for years now, ever since my weight began creeping up following the birth of my children and the diagnosis of a fairly catastrophically underactive thyroid.
The latter has caused me all kinds of problems over the years, from chronic hair loss to sore joints and mood swings. I have found ways of managing many of the issues, but the one thing that I can't seem to shake is the weight.
And I hate it.
Part of the problem is a hectic existence. Another is lack of motivation. Another still is my fondness for the odd drop of vin rouge. But there's something else, something that may resonate with other working mums: I'm always last on my own to-do list.
That is about to change. The next few days are all about me.
Diary: Day 1
I arrive after lunch, having stuffed myself with a final Marks and Spencer egg sandwich on the motorway. I ate it greedily, like a smoker sucking down their final cigarette. If this thing works, it will be my last.
The room is huge, beautifully decked out in shades of eau de nil and fashionably distressed furniture. The windows look out onto neat parkland, an oasis of peace save for the enthusiastic chirping of birds.
After a brief tour of the facilities (spa area, pool, restaurant, a very chic boutique stocking tempting bathing costumes), I am called for my first appointment, with the nurse Mary.
Mary is a twinkly lady of middle age, exactly as I imagine all the best nurses to be. She takes down some basic medical history, then pricks my finger for my blood tests. While we are waiting for the results (they have some whizzy piece of machinery that does it all in a matter of minutes), I am handed over to Ravi, the gym manager, whose job it is to determine my body composition.
I try not to look as 15 grand's worth of high tech machinery scans my body for fat, muscle and liquid content. But I can't help it; by the time we sit down again, I know the score.
"You are actually quite fit," he says, sounding surprised. "And you have good muscle mass. But you also have 20.8 kilos of excess fat. And your waist to hip ratio is very poor."
Great. I am officially a barrel of lard.
My next encounter is with Moore herself. We go through my bloods: again, surprisingly good. Cholesterol fine, liver fine, blood sugar fine, iron fine. Pretty much all in the middle of the scale. We talk through my diet, and she nods in approval.
"So why am I so fat?" I ask her.
Simple: my carbohydrate intake is too high. What I'm getting right is being cancelled out by eating the wrong sorts of vegetables, fruits and grains. Which means my ability to burn fat is very poor. What the week's programme will do is redress that imbalance; I will eat simple proteins, no grains, no dairy, no starchy veg. Sugar - the great enemy - will be entirely absent. And there will be two fasting days, consisting of no breakfast, lunch and broth at 7pm.
It's shaping up to be a long week.
(Stephanie told me two fascinating things I never knew about sugar. Firstly, brain scans show that sugar is an opiate stimulant, lighting up the same area in the brain as heroin.
And secondly, a normal, healthy human cell has two glucose receptors that respond to sugar. A cancerous cell can have anything up to 87. Highly addictive and a cancer-magnet. Why is sugar still legal?)
20.8kg or not, I am hungry. Half six is the official start of dinner, but I don't want to seem too keen, so I head down at around 7pm, wearing the blue wristband that indicates that I am on the Regime. Soon, I am joined by three fellow diners.
First, a cup of bitters and a small pot of pickled cabbage to aid digestion. Then a two course meal: a starter of salmon and avocado (delicious), then pork with vegetables (really very delicious). Tasty and filling, which is just as well because tomorrow is a fasting day: nothing until lunchtime.