Yesterday still seized solid so I cut the exposed and chewed threads of the retaining bolt (didn't want them to tear the thread in the brake lever housing on the way out). Several careful blows with a hammer on the bolt head and finally I was able to tease it out with an allen key.
My problem now is getting the bush out of the brake lever, it's stuck fast. I have applied gentle heat and then tried to drift it out but it wont budge. I think the bush is brass so would drilling it out be a better approach?