THE departure of Sir William Purves from the Executive Council this week marks the end of an era. As the taipan of one of Hong Kong's oldest institutions, Hongkong Bank, Sir William brought a wealth of knowledge to the Governor's inner cabinet. His experience and understanding of the territory will be difficult to replace. As much as Sir William will be missed, however, his departure will afford Chris Patten the opportunity to further imprint his stamp on Exco. Will the Governor fill the seat with a liberal who is supportive of his policies or will he continue in the tradition laid down by his predecessors, because of the unique position of the Hongkong Bank, of appointing its chairman? A liberal appointment would be tempting. With his political reform package under siege on many fronts, Mr Patten needs all the support he can muster. However, there are not many places Mr Patten will find capable liberals outside of the Legislative Council or legal profession. With two barristers already on Exco, another - however liberal-minded - would appear excessive and lay Mr Patten open to charges that his top advisory body is not broadly representative and lacks the variety of expertise needed to ensure the Governor gets good based advice on a range of issues. The arguments for Mr Patten to bring in a senior banker are more compelling. As able as his Financial Secretary may be, Mr Patten still needs the input of the private sector in advising on the territory's lifeblood - money matters. So a private sector financial expert - a banker - it should be. But which banker should it be, and from which bank? Although the Hongkong Bank has long enjoyed the Governor's ear in Exco, the time may be right to break with this tradition and consider offering the seat to one of the other big banking institutions. At a time when localisation is a concern, the ideal candidate could be a respected local banker and not necessarily an expatriate. On the official side, Mr Patten has yet to fill the vacancy left by John Chan Cho-chak, who chose to retire from government early. In keeping with his preference to bring in younger advisers, Mr Patten will want to appoint a local high flier. Of course, choosing candidates to sit on Exco could turn out to be a dilemma for Mr Patten. Rather than having his pick of the best and the brightest that Hong Kong has to offer, he may find there are those who prefer to keep their distance from him and his policies out of concern that too close an association will jeopardise their links with China after 1997.