I'd second the comments by IAN-UK and adder about the HI Continental.

We stayed on a prepaid 2-4-1. Reception was fine and we had a good upgrade. Reception manager spoke great English (our Portuguese is poor). On checkout, they tried to charge us for the 2nd 'free' night. No English this time. In the end, I didn't pay (of course) and left a written message - we needed to leave for the airport.

Of course, points didn't post. This required more than one exchange of e-mails to resolve.

Apart from that, a delightful experience.

In addition to the transport options mentioned, this hotel is on a bus route from the aiport, though with Lisbon taxi rates being reasonable, you may not bother with the bus.