Hong Kong income and tax

First time I've ever posted a question on here, so please be gentle with me!

I have just taken on a tax return client who lives in the UK, has a UK pension but works as long haul cabin crew for a Hong Kong company. The Hong Kong company pays my client gross without any deduction of tax whatsoever. I am told by the client that his colleagues who are in a similar situation tell him that it can take up to two years for the Hong Kong tax authorities to ask for the tax. In the meantime how do I complete the client's UK tax return? I'm assuming there must be a double taxation treaty between the UK and Hong Kong (haven't checked that yet) but if we don't yet know what the HK tax will be what do we put on the tax return? Will an estimate be sufficient and if so what happens if the estimate turns out to be too high or too low? Do we go back and file an amended return in a couple of years time when the actual HK tax is known?
Paul Sherrington