"Are you suggesting... that the Brits are keen at maintaining the residual of their once great empire ...by keeping their grimy paws on the North? I do not find that very convincing, because the North looks more like a liability than an asset"
The relationship between economic motive and British policy in Ireland was rarely direct after partition.
Those who did try to develop a Marxoid analysis of the Six Counties that explained British policy as a direct reflection of a desire to expropriate super-profits through the export of surplus capital (like Repsol) quickly found that no such thing was happening, and usually drew the false conclusion that British capitalism's goals were benevolent.
It is true that the effect of the partition was to keep the industrialised North in the UK, while the agrarian South was left starved of technology - but even that was more due to the authority the Crown had among the protestant Scottish settlers in Ulster, than it was to deliberate policy.
Primarily it was politics that made the British intransigent. They understood that the revolt in Ireland was dramatic example to revolutionaries throughout the Empire. I've often been surprised at the extent of the IRA's moral influence - rebels in Burma read Sinn Fein literature in the forties, Ho Chi Minh wept at the news of the Sinn Fein Mayor of Cork Terence Macswiney's death on hunder strike (if I remember the story right, Ho was working as a dishwasher in a hotel when he heard it on the radio). In the 1980s Khomeini's government re-named the street where the British embassy is in Iran after the hunger striker Bobby Sands.
The Republicans' campaign went further than overthrowing the Empire, under the Act of Union, Ireland was incorporated into the United Kingdom. Independence threatened the state itself.
Britain's primary goal in northern Ireland has always been the defeat of the nationalist movement. Though it is not the hands-down victory that some of the more lumpen unionists hoped for, Adams has delivered the movement into British hands.
The decisive - and utterly demoralising - demand that Sinn Fein has made in recent times, is that the British government act as an honest broker, or an agent for change in Ireland. After all, armed struggle was in the end only one tactic, not a principle, but the positive part of 'Ourselves Alone' was precisely that the republicans did not put any faith in the British state to act for them. Now they do.