Well I've just looked at this again and really the CAAD8 is set up more for competitive riding and although my LBS can get them cheaper at the moment (because everyone wants the Synapse) the way the gearing is set up I wouldn't get up the hills around here until I've lost a stone or 2!!
The Synapse Sora however looks much more my kind of ride albeit £50 over budget. So the plan is now to patch up my current bike and see if it will take me through the next few months and see how my weight and fitness goes then reassess whether the CAAD8 or Synapse is still the right one and maybe get the money together to go Tiagra instead.