Simon: The water leak sounds like the expansion chamber air pressure is not set right - there should be a schrader type valve on it somewhere - stick your tyre pressure gauge on it and see what it says - should be 0.5-1 barg. Lighting at full load doesn't sound right, perhaps they have installed the wrong board. If there is no diagnostics on the board, difficult to tell why it won't light. It might be that the ignitor gap is set wrong (mine does this, gap closes up and then it won't light - have a new one to stick in, not got round to it yet, but have had to re-set it last two mornings....!). There should be a re-set function somewhere - often another position on the control knob, or maybe a separate knob - anything in the instructions about it?
Jim: AGAs and Rayburns are quite different really. An AGA has a small burner (3kW in a gas one), but is designed to store a lot of heat (they weigh something like a ton and have to be assembled on site), which is used for cooking - which is AGA's primary function. They can also be fitted with small boilers, to run a couple of rads, but this is secondary. A Rayburn is much lighter (they are installed in one piece), have a bigger burner, and are design primarily as CH boiler, with cooking on the side. Rayburns are much more amenable to stoking up a big fire to generate high heat for cooking, whereas with an AGA you need to give it written notice if you want it hot

Mind you, either will become the centre of your kitchen! Mind you, the cost will make you weep - a 2/3 oven gas AGA will be the thick end of £10k installed

We had an old oil fired AGA OB (A rather odd design from the early 70's with square lids), and my parents have now got a gas fired one in their house (which they installed when they rebuilt it in 2003) - I'm rather a fan, but SWMBO isn't - and anyway - TEN GRAND
Stoves: we've got a
Charnwood Cove in our house - I installed it in place of the back boiler I took out in 2007. It's great! Interestingly (well to me anyway....!) Charnwood are Isle of Wight based, and we had a stove from them when we lived there a bit over 30 years ago - then it was one guy knocking them out in his garage effectively - they are somewhat larger now! It's an excuse to have a chainsaw anyway!