https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:77098190-1 - Fallopia baldschuanica (mile-a-minute, Russian-vine) × Reynoutria japonica (Japanese knotweed) = × Reyllopia conollyana (Haringey knotweed, railway-yard knotweed)

Should be in Subfamily Polygonoideae -> Tribe Polygoneae -> Subtribe Reynoutriinae like both parent genuses.

I add an asterisk because it's already on iNat as an inactive taxon under the old name Fallopia × conollyana https://www.inaturalist.org/taxa/429265-Fallopia---conollyana and with both common names in place (though the Haringey one should probably be prioritised in the UK).

Known as a naturally occuring hybrid e.g https://alienplantsbelgium.myspecies.info/species-list/%C3%97reyllopia-conollyana

I've sent them an email asking about it. it seems like a mistake to me to list it as artificial (especially given it's rarely known to self-sow) but will see what they say

taxon looks good (though the genus name isn't capitalised on the species), cheers - not sure if it's possible to set it as the 'successor' taxon to the old one like with merges, but it never had any observations anyway https://www.inaturalist.org/identifications?taxon_id=429265&current=any so if it's not it doesn't really matter too much

did get a reply earlier today --

The parents are not native so it cannot be a natural hybrid. It may be a spontaneous hybrid but that happens in gardens as well. So it is artificial in that humans brought the parents together outside their native range.

I assumed 'artificial hybrid' meant 'cultivated hybrid' rather than something so wide - so whilst it's not an error that it's listed as an 'artificial hybrid', that doesn't mean that it cannot occur 'naturally' - since Japanese Knotweed is slightly frowned upon in cultivation these days I doubt it's deliberately been planted anywhere.

perhaps worth making a trip to Railway Fields some time next year and trying to find the original - the only open photos of it I can find are the low-res ones on the Belgian Aliens site. it's a very rare taxon so might sit empty for a while if nobody else does!