Zener diode?

Based on the use throughout, odds are good they're plain vanilla 1N914 signal diodes.

What's the problem with the board?

One diode was damaged. The burner wasn't coming on at all. N all testing via Rayburn method came up with replace PCB board. But if it is just the faulty diode all is good. Otherwise its nearly £700 for a new board!

A few thoughts - there's little chance of finding a schematic from what I see of Rayburner online. It was 20+ years ago when I was dealing with their burners regularly (or more like 30?) and documentation was hard to come by back then. I'd keep trying and maybe contact the Rayburner people about a schematic.

Most of these style boards don't use anything exotic diodewise (or otherwise). I'd start by removing and checking the suspect diode, if it acts odd, replace it with a 1n914/1n4000 series diode if it measures over .8v foward voltage or shorted (99% of the time a bad diode is shorted if it's physically intact) If you're sure it's bad, replace it, trace the circuit to make sure what it's driving isn't bad and fire it up. Odds are good you'll have it fixed.
(1n4007 diodes are the higher voltage flavor and sub for most small diodes/rectifiers 95% of the time - they're cheap and readily available)

Next verify the solenoids etc on the bad burner are ok. Sometimes you can swap leads between good and bad burners to identify problems.

The next step past that is to see if the working burner can yield info about the bad one. Usually the circuits will be duplicated for the most part on the 2nd burner.

From there, it's a matter of working out a schematic, concentrating on the likely trouble points. Narrow down the problem, at what point in the start sequence does it abort. That will help narrow down the cause of the fault.

The last step is to replace the board with a scratch built one or one for a similar machine. You probably won't have to go that far, but sometimes it's the only option (especially on large, special or antique boilers/exchangers)

Usually those boards are a burner start-up sequencer/flame out protection device only and are largely interchangable. (thermostatic controls are usually separate)
Since this is a safety device, don't get into anything over your skill level, but usually obsolete burner controls are reproducible/repairable with a little re-wiring and maybe replacing a sensor or 2.

I'd have a look of it, but it seems we're on opposite sides of the big pond. (I'm on the east coast of the US). A good service tech can fix that much cheaper than the stated 700 pounds ($1500 US - I'd guess)

On 1 of the diodes I can just make out 1N but not very clear. Can't see much else.

The board layout makes me think that the diodes are used in an odd way. There are very few tracks to the diodes, and none to the space for the "HS1" that goes between them. Is there a connection directly between each pair of diodes? I can't see from the circuit. Also, other diodes on the board have a marked corner on the rectangle around the diode. Do the burned ones have the same corner mark?

Is there something special about that bit of the board? Boards are normally rectangular unless someone has a good reason to make them a more complicated shape, and the burned diodes seem to be positioned where they are for a reason. The metal plate the the board is mounted on seems to be bent up to be a heatsink for V2 (and possibly V1 and BR1 as well, maybe on other options). However the burned diodes are a long way from the other components.

I am wondering if HS1 was a heatsink on some version of the board, and on yours, Aga thought they could get away without it.