The biggest car auction auction company in the UK is the US-based firm, Manheim, who've got a storage site on the outskirts of London Colney. They will frequently be looking for new trade-platers, especially during the winter months when hitching is much less pleasant and far harder. (Drivers tend not to see hitch-hikers in the pouring rain, at 6.00am, when it's still dark).
I assume that someone at Manheim's London Colney depot has tried to work out which type of people like being outdoors but who won't be able to find much work during the winter months, which would explain the reference to gardening.
I worked as a trade plater, for an independent company, delivering mainly to auction sites (including Manheim at London Colney), from February 1999 to September 2001. I was required to sign a contract stating that I was self-employed (which meant that, as far as the company were concerned, I was only 'working' when driving the vehicles and not while travelling to collect them). In my final tax year (2000 - 2001), I worked 50 weeks for never less than 60 hours per week. (Typically, around 65-70 hours). My total pay was �8600 but, because you can't hitch everywhere, I'd paid out �1500 in fares. So my pre-tax pay was actually �7100. That works out at not much more than �2 per hour (which, at that time, was roughly half the national minimum wage). However, I knew many of the Manheim trade platers. They did earn substantially more than me but they still worked very long hours.
Fun job - lousy pay!
Chris