Take a look at The Villa Agency

The Villa Agency are specialists in Portugal and nearly all their villas have fine pools. 
They are one of the few remaining privately owned villa rental companies specialising in Portugal. They are fully bonded so your money is 100% safe. 

I have spent some time browsing round their site at http://www.thevillaagency.co.uk/ and their selection of villas looks superb. Also their attention to detail and honesty are refreshing. This is what they say on their front page:

"When you telephone us you will hear a friendly voice, not a recorded multiple choice announcement. We love talking about our villas and have all the time in the world to assist you. Among our staff are mums who take their children to our villas and are happy to discuss each property with you, helping to choose the right one for your family. We can provide a free cot and highchair if required.

We believe that our personal and old fashioned approach to service is the reason why so many clients choose to book with us year after year."

Prices are competitive and if you are looking for an out of season bargain there are some big villas (sleeping 6 or over) that are under £1000 per week.