Does this sound like my alternator going south?

[MG68]

My new to me V70 Classic 2.5D is going great @ 117k miles.

Well it is, until I get to 65-70mph, when the battery charging light comes on solid red. Most of the time this week it has been wafting around less than that speed and no warning light.

It starts first time, no dim or brighter headlights, or anything else strange.

It's booked in at my local garage next week, but on the face of it, does this sound like an alternator going down?

Not sure when the battery was changed, but it is a 75 mha.

Never seen this before - has anyone had it happen? Also, what would be a reasonable cost for a replacement alternator / aux belt for this engine?

Many thanks in advance, Mark.