Now the question is why were they there?
The first two Dan-Air One-Elevens came in March 1969, from American Airlines. They were a hurried purchase following the bankruptcy of British Eagle the previous autumn who had a major holiday flight contract with Lunn-Poly out of Heathrow, which Dan-Air had moved quickly to pick up. Unusually for Dan-Air they were not based at Gatwick, but at Luton, where I always thought up to this time they were rare visitors and did not base aircraft there (you others here probably know more on this). G-ATPL joined them in October 1969, after standing a year since the Eagle failure. In those days there was not a lot of winter holiday charter work and it was common for aircraft to be parked for extended periods through the winter.
Dan-Air had their main engineering base at Lasham, of course, and also a substantial base presence at Gatwick. Who looked after them at Luton in their early days there ? Did they set up their own maintenance base ?