Ok - excuse my ignornace - 2 tanks in the lost.

One is the cold water feeder - its far larger than the expansion tank -
which is quite small and the one that is creating the drip on the
outside of my house.

The colder water tank eg - fills the toilet up etc when pulled

Second floor - hot water cylindar

Note - expansion tank only over flows durring the timing when the hot
water is on.

So - the cold water feeder tank - that level should be quite high?

Scott