Halfords have my forks now. Their quality control department have studied them and concluded: Metal Fatigue.
The forks are no longer under warranty and Boardman don't make this model of bike and therefore don't make the forks so they can't get a direct replacement, which to be honest I did expect. They have however offered some replacement forks direct from Madison. They appear to be some standard carbon bladed road forks, and they are offered to cover half the cost of these £60, which I guess is fair enough?
The other option (which i'll probably go down) is buy some new forks and then get Halfords to reimburse me half cost, given I need mudguard eyelets and clearance for a mudgard I think i'll get some CSN Winter ones from Ribble (£80) I had these on my Ribble previously, and to be honest they were ideal.
So I think that's the final part of the story? I'm still not 100% sure on the metal fatigue conclusion though...