MIL is getting a new Combi (yes I know!), and has been offered a Viessmann or Vaillant.

The Viessmann seems to get good review – what says the STw plumbing / ch hivemind?

no experience of viessmann but i can recommend vaillant or worcester bosch

i'd also advise steering clear of most other makes of combi

I'm about to change ours. Plumber told me, if you're staying in the flat you can't go wrong with Viessman, Vaillant or Worcester. If you're moving in the next couple of years: Vokera non-condensing.

I'll be getting the Vokera.

AFAIK Vaillant always get good reviews – better than Worcester Bosch on DIY forums.

They both begin with V but the ones a steel old dog the others a carbon race bike. Buy a Vaillant.