I have been doing a little more digging on this and have found other posts on dimming spot bulbs etc. There is a Vellman Dimmer kit (K8064) available that can be controlled from a 0-10V DC current this would be easier than me messing around with AC given my current novice level and they are relatively cheap. There were warnings on the posts to add a 'big capacitor' in to the connection between the Arduino and the K8064 if using the PWM port as the K8064 doesn't like the pulses, however it doesn't specify how big ?
As my Arduino Mega is 5v dc I was planning to amplify to the required 10v dc using a simple op amp non-inverting circuit (741 op amp and two 10K resistors - yet to test this). My question is do I have to control this from a PWM port or can I use one of the other ports on the Arduino, and if so is the capacitor required still.
In addition I am using a TMP36 sensor to measure the temp in my test circuit can anyone advise on the most accurate ones to use I am monitoring temps in the range of 60-120F also can anyone that can advise on the best sensor to use for measuring humidity, as yet I haven't purchased one of these yet.