I think the whole light unit has to be removed, Pete.
It’s a bit of pia, but doesn’t take that long. From memory, remove front ‘Hymer’ panel (four screws) to reveal a headlight unit bolt (remove) that holds headlight unit in, and remove the other, more visible bolt. The headlight is then only held in by a stud that’s just pushed into a grommet in the wing – pull and wiggle it until it pops out. Invert the headlight to gain access to the bulbs.
It may be possible to do it another way but I don’t know of it. I’ve changed my headlight units once and I’ve also had a young lad at Halfords change a bulb this way (one cold, sleety-rainy evening in winter!). So you could let Halfords do it for you, but it will cost you about £7 extra.