Old oil-fired to new NatGas boiler swap - recommendations

[Symptoms]

I'm buying a house with an old oil-fired CH boiler (gravity/unvented system) and want to swap it for a Natural Gas job.

The gas provider's doing a deal at the moment - running the gas from the street, installing meter ... all for 'free' ... when a new boiler is installed by their preferred (off a list of independents) Plumbing/Heating firms. But they are only allowing, Baxi, Ferroli, Glow-worm, Halstead, Ideal, Ravenheat, Vaillant, Vokera, Worcester boilers.

It's a big old house, so the boiler's going to need plenty of grunt (probably circa 35KW) and I don't want to mess with the existing pipework/tank/hwcylinder (try to keep it gravity/vented). Obviously, I'll be getting in 2 or 3 firms to advise & quote, but I'd like to have a bit of background on the merits of the above manufacturers' tackle. So, I'd be grateful for your comments about this ... but keep it 'clean' (nothing libellous) to avoid the clutches of our learned friends.

 

[spacethegas]

out of that bunch my choice would be Vaillant.

 

[BigBurner]

out of that bunch my choice would be Vaillant.

Glow Worm are cheaper and owned by Vaillant and have many of the same parts.

 

[Mike2007]

I have natural gas and would gladly swap for oil, WHY?
Because I would run it on cleaned waste vegetable oil I collect for nothing. If that sounds a grotty/dirty way to run a boiler, then maybe in two years time after NG has risen to an totally unaffordable price to run water heating it won't.
Sadly no work for Corgi engineers there though!