Isn't it still the case that Sky are still restricted to three slots on COM5? It of course didn't stop them from acquiring capacity on other muxes.
Bit of an odd one, this, and I'm happy to be corrected on it, but:
The licence states that the multiplex "shall implement" the carriage of three Sky channels, and that at least "one quarter" of the available capacity is used for services not connected with Sky.
So the limit was never strictly three slots, just up to 75% of the bandwidth. This is how Sky proposed to switch to MPEG4 and broadcast more channels in the same amount of space when they were thinking of launching "Picnic" (which eventually morphed into what is now Now TV) while remaining within their licence terms.
Of course, when a multiplex could only carry four channels, the situation was much more straightforward!