i have just found your question and guess that by now you have had professional advice already.Here is the solution anyway- the existing switch is an `intermediate`switch and not just a 2-way switch ie it is one of three or more switches controlling one lighting circuit. you will need to exchange the new 2-way switch for an intermediate switch then connect as before . reds jointed, yellows in L1`s and corresponding blues in L2`s
monty the hamster, October 2006