This is commonly used to describe a lady of a certain age and of a certain nocturnal habit and who is also an inhabitant of the Kelso / Selkirk regions.
May not be the original provenance, but I have a Viz Annual from a couple of years back, which had the results of a "Borderline Boilers" Poll, in which readers were asked to nominate their favourite Borderline Boilers or "certain angle stunners". Top of the poll was Gerry Halliwell.
Of Helen Mirren (No. 10 in the poll), they said "Voted the sexiest woman in the world, back in the sixties, the intervening decades have battered her once rivetting good looks. However, time has not withered her enthusiasm for getting her kit off, which we suspect may account for her prime position in our top 100 Blart Chart."
I won't tell you what they said about Charlie Dimmock; it's too cruel. /forums/images/graemlins/crazy.gif
Exactly; you've got the idea! Not in the Viz top 100, but should have been.
Another example: Carol Vorderman (No. 8)
"Cambridge-educated Carol reaches no. 8 in our countdown of the 'Top 100 Happy Shopper Beauties' and with a third class maths degree and second class looks, she adds up to a first class borderline boiler."
P.S. Helen Mirren is definitely a proper bedwarmer. Like a good red wine, she just keeps on getting better as she matures. /forums/images/graemlins/grin.gif