Yeah, so Halfords took one look at the two rusty and seized screws in the clear plastic that covers each bulb, and said 'no way'.
So I took it to Migliore. Firstly, they removed, with difficulty, the the two screws. You would think that this would simply release that bit of clear plastic, giving access to the bulb. But, no! Removing these screws just weirdly loosens the whole light assembly, without giving access to the bulb. In the end they had to dismantle basically the whole boot lid to release the number-plate light unit and be able to change the bulbs. (Might as well do both while they are in there.)
Worse still, it has been an intermittent fault for about the past four months, and on the day I took it to Migliore, the bulb was working fine. So basically I paid about £100 to replace two working bulbs.