"In expressing views on the position of BT [the intervener], we are not allowing the indications we are about to give to harden into a rule, but they do express our view in general on interveners in the situation of BT. The general position, as far as the Tribunal is concerned, is that the costs of an intervention will very often in justice be allowed to lie where they fall. It is true that in some cases it will be proper to make orders either in favour of or against interveners, but in our view there should be no general expectation that a successful intervener is necessarily entitled to its costs."