I had just posted my question and then continued reading down the page. All the comments and answers relating to similar questions made sense. My husband and I tried it - unclipped the plastic cup, drained out all the water. When we switched the boiler back on IT WORKED!!!!!! The flame light came on and the hot water was back.
This website is amazing. Thank you all. Just saved ourselves lots of dosh!!
CJ
September 2012