All the responses are very helpful but I am inclined to think that a spa needs some form of water jet(s) to work on those sore muscles.
I read reviews on the internet from people who had used a 12 volt portable pump in the bath which is held in place by suctions.
There were two problems with these types of pumps.
1.The pressure is very low primarily due to the small capacity of the pump.
2.The suctions keep loosing suction.
I then checked out other types of pumps and what appealed to me was a submersible pump used mainly for fountains.
I bought a Sunsun JP-068 submersible pump ( for $33 plus postage from ebay ) which is small (2.0kg ) and yet powerful ( 240 volts,75 watts, 3100L/H ) enough to give me all the power that I demand from a spa yet.
I have now already used this pump over the last two weekends and feel very satisfied with my purchase.
In addition, this pump has an adjustment for the pressure flow but I have to set it to maximum for what I want.
If I had to buy another pump I would this time purchase the JP-069 model which is slightly more powerful.