But why a milk-float? "It does less damage to our lungs than anything else I could think of," he replies. The pedal bikes with trailers they used previously were limited. They couldn't carry ladders, and hadn't enough flat space to ferry plants about. The float has a battery, charged up overnight, and a big, flat base for plants. "When we stop at traffic lights, people try to buy stuff off the back of the float."
A big blackboard on the wall of their office - one of the waiting-rooms at the old Primrose Hill railway station - shows what the Garden Rescue Service is up to at the moment: landscaping, design, lighting, turfing, fencing, trellis, clearance, pruning. They provide plants and build planters to put them in. They make pond covers and storage bins from recycled timber. They have a little wildlife park at 3B Goodsway, London NW1. They pick up garden rubbish and deliver it to Camden's recycling centre, where it is used to feed a power station. They advise on "integrated pest management" (using bugs to catch bugs) and run a small garden library for the use of their clients.
"We're not just a maintenance service," says St John firmly, tugging his forage cap even lower down over his eyes. "We like to feel we are giving these gardens back to their owners, for them to enjoy and work in as well. We help with things they can't tackle." Such as runaway wisteria, overgrown rambling roses, leaking ponds.
City gardens have their own special problems: earth that is sometimes little more than ground-up bricks and mortar, drought, shade, cats - especially cats. And no side entrances. Only 20 per cent of the gardens that St John and his two team leaders, Leo and Josh, go to have direct access. The rest of them can be reached only through the house.
That means quantities of polythene sheet to cover the hall carpet, owners clucking anxiously, all rubbish packed hygienically in black plastic bin-liners. "Oh yes,." says Leo, nodding his fair hair extensions (they are pulled back into a pony-tail). "We are very skilled at going through houses."
St John's "moment of epiphany", as he puts it, happened on an allotment in Scotland. He funded his way through the University of East Anglia by working as a chef, then wandered northwards, settling finally with a sister in Edinburgh. He got the allotment to grow herbs, became good at growing things, then picked up work as a jobbing gardener. But the cold drove him south again and here he is in Primrose Hill, with - he seems slightly surprised that it's so - his own business.
The typical client, he says, is aged 28-50, professional, short of time but interested in the garden. "Publishers," puts in Leo. "Yes," says St John. "Lots of publishers." And is it easy to agree with a client what needs to be done in the garden? "Oh yes," says St John. "But we don't always agree amongst ourselves," adds Leo. "We'll happily bicker for hours" acknowledges St John. "Josh won't give house room to hibiscus, but he likes bergenia," Leo says the word with incredulity. "A healthy exchange of prejudices," explains St John..
Greenness is important to them. And recycling. As few pesticides as possible. No petrol. No landfill. But with the vague, incoherent leaning towards the organic that often affects town gardeners, it's also good business. The little wildlife park they have made at 3B Goodsway in the shade of the world's first gasometers has environmentally-friendly ponds and walkways, but Garden Rescue also sells plants and planters there.
And they don't spin a web of mystery around garden design. "Look," says St John. "Design has to come from function. A London garden has to be an extension of the house. So you just have to ask people what this room is for." "Courtyard and curtain," says Leo. "Courtyard and curtain," agrees St John. I begin to feel this is a mantra that has passed me by. St John explains. The courtyard is the outside, living bit, the curtain the screen of plants that protects the courtyard from being overlooked, or screens off a bad view. They are the two most important elements in any design.
My trip ends with a ceremonial visit to the milk-float, parked on the far side of the railway bridge. As it sits there, bug-eyed, unthreatening, with its cargo of rakes and compost, wisteria prunings and winter heathers, you realise what a clever piece of PR it represents. Leo and Josh are relieved that St John has finally decided against adding a couple of three- wheeled vehicles to the transport fleet. "Inherently unstable" is the verdict; "inherently uncool" the sub-text.
The Garden Rescue Service, 123 Regent's Park Rd, London NW1 (0171-586 7714), is open Wednesdays and Fridays.