Ashworth Frazer Ltd v Gloucester City Council  UKHL 59,  1 All ER 377, in which Lord Bingham stated:
64. " When a difference is to be resolved between landlord and tenant following the imposition of a condition (an event which need not be separately considered) or a withholding of consent, effect must be given to three overriding principles. The first, as expressed by Balcombe LJ in International Drilling Fluids Ltd v Louisville Investments (Uxbridge) Ltd  1 All ER 321 at 325,  Ch 513 at 520 is that—
65. ‘a landlord is not entitled to refuse his consent to an assignment on grounds which have nothing whatever to do with the relationship of landlord and tenant in regard to the subject matter of the lease.’
66. The same principle was earlier expressed by Sargant LJ in Re Gibbs & Houlder Brothers & Co Ltd’s Lease, Houlder Brothers & Co Ltd v Gibbs  Ch 575 at 587,  All ER Rep 128 at 134:
67. ‘ in a case of this kind the reason must be something affecting the subject matter of the contract which forms the relationship between the landlord and the tenant, and it must not be something wholly extraneous and completely dissociated from the subject matter of the contract.’
68. While difficult borderline questions are bound to arise, the principle to be applied is clear.
69.  Secondly, in any case where the requirements of the first principle are met, the question whether the landlord’s conduct was reasonable or unreasonable will be one of fact to be decided by the tribunal of fact The correct approach was very clearly laid down by Lord Denning MR in Bickel v Duke of Westminster  QB 517 at 524.
70.  Thirdly, the landlord’s obligation is to show that his conduct was reasonable, not that it was right or justifiable. As Danckwerts LJ held in Pimms Ltd v Tallow Chandlers Co in the City of London  2 QB 547 at 564
71. " it is not necessary for the landlords to prove that the conclusions which led them to refuse consent were justified, if they were conclusions which might be reached by a reasonable man in the circumstances "
72. Subject always to the first principle outlined above, I would respectfully endorse the observation of Viscount Dunedin in Viscount Tredegar v Harwood  AC 72 at 78, that one ‘should read reasonableness in the general sense’. There are few expressions more routinely used by British lawyers than ‘reasonable’, and the expression should be given a broad, commonsense meaning in this context as in others."