The reflectors on my front fog lights have corroded. Has anyone used replacement LED bulbs with built-in lenses? I'm wondering whether I can use them instead of replacing both light units.

Ta!

The beam pattern from a foglight is a combination of reflector shape and lens pattern. They generally have a shield in front of the actual bulb to prevent direct lighting. So an LED with its own lens would possibly not show much light at all, or if there is no shield, would display the wrong pattern.

To further confuse the issue, most sellers online are unaware, or uncaring that this actually a difference between fog & spot lights and use the terms interchangeably.

+1
The front fog lights on the Disco 2 I presume you are talking about are the round ones set in the front bumper.
AFAIK, the front lens and the reflector are glued together as a single unit and not intended to come apart.
Since there's no pattern or fluting on the lens, it's fair to say that the beam pattern is a product of the reflector shape, consequently the type of bulb and the actual position of the light source (the filament) is crucial for the correct flat beam characteristic of a fog lamp. Another problem with some LED "replacements" is that the total length of the bulb is too great to fit into the lamp housing, particularly noticeable with the H3 bulb.

Thanks. I'll use the more conventional approach of replacing the units and bulbs. There are some new to market that cost £80 a pair rather than £50+ each.

It's a pity replacement LEDs are such a minefield, I'd like to move towards replacing all my bulbs with them.