I was able to access it by removing the third brake-light and bending the roof-lining down at the back. This revealed the underside of the aerial base, which was held in place by a 15mm nut. I had thought the entire roof-lining would need to come down, but got away without doinh that so it is worth noting that this is an easy way to access the underneath of the 'shark-fin' roof antenna.
I had expected the base to be a 'stick-on' type, with a layer of adhesive underneath like a badge, but found this baby had been stuck down with a liberal amount of strong silicone-based sealant. A bit overkill considering the size and weight of the aerial-base, but I guess unmarked Police cars must still get a bit of vandalism... Sadly, the plastic top broke off so I was left with a snapped base and a hole in the roof of my car, meaning my chances of turning back were out of the question and I would have to see the removal through come what may.
So, like I say, I wish I had left the aerial-base on for the time being! As if the high-spot round the hole created by tightening the nut wasn't bad enough, even the gentlest of prising has left marks in the paint / lacquer... nightmare. But, alas, here we are so the question is what to do about it now?
A full paint repair would involve removing the raised rim from the hole, bridging from underneath with fibreglass-mat and P40 fibreglass-filler, keying a small circular area around the damage, smoothing off from above with body-filler and then somehow priming, painting and lacquering the affected area in sympathy with the rest of the roof... Whoa, this has created quite a lot of work here. For the time being a square of PVC-tape will have to do! Watch this space as the weather improves!