not really circular refs

For example, I want to be able to enter either amount in Pounds or Euro, and to simplify lets say £1 = €1.618034 (or €1 = £0.618034) {doncha just love the golden ratio?}

Cell A1 = B1*1.618034
Cell B1 = A1*0.618034 (or A1/1.618034)

Either I enter currency in Pounds in A1 or in Euro in B1 and the appropriate conversion takes place. How do I avoid circular references before any data is entered?

I've tidled around with ISNUMBER but that doesn't seem to work. There must be lots of similar cases e.g. net + tax = gross and you want to be able to have both formulas in place and overwrite when you enter real data.

Thanks for any pointers, Dave.