James! I thought I was only seeing one length of elytral hairs, but I guess not. The antennae are certainly quite long. Let me know if some specimen pics will help with the species ID.

You can see a few longer hairs sticking out from the elytra in a few places. But yes, the long antennae, and unusual underside colouration, are pretty strange if it were Scymninae.

so it would seem that forestieri is the likely species here?

It is fundamentally inseparable from R. ventralis, which has been part of introduction programs in the past. With that in mind I'm not sure if it is more likely to be a sudden extension of forestieri (recently known only as far south as northern California, that's quite a distance away!), or remnants of an old introduction program for ventralis.

I have the specimen - can I send it to someone for a genitalia dissection, for example?

But I am not aware of a reference comparing the two species. It may depend on whether it is a male or female as well.

this article suggests females, at least, of ventralis and forestieri can be separated by dissection:

https://www.researchgate.net/publication/274194847_Rhyzobius_forestieri_Mulsant_1853_Coccinellidae_an_Australian_ladybird_new_to_Britain_from_the_Wildlife_Garden_of_the_Natural_History_Museum_London