Hi everyone.  
Thank you all for your thoughts.  We have been over it quite a few times and have decided to get the pool.  Lara, I have a client who is a pool builder who will look after us so I don't need to worry about being ripped off.  Our 8 year old is a recreational swimmer (represents his school in GPS swimming but I refuse to let him do club), so a small pool isn't really an option.  
We are going to go salt and will have solar.  We rented a house while we built with a pool and we could only swim in it from November to March where as our 8 year old swims in our neighbours heated pool year round.  It should also get plenty of sun as we have 5000m2 and only 2 trees.  Burnsy, I'll be chasing some tips down the track but we only had one issue while we were renting and that was due to the weather.
Stan, I know we only live once and we aren't giving up camping... just changing our style a little.  I don't want to leave my family and more debt than I need to if something should happen to me.  A pool in summer will give more family time and less TV time after dark and that can only be a good thing.
Expanda, our boys a 8 and 2 and someone very validly pointed out that every year we delay putting one in, is another year that it won't get used.