I've just moved into a property and discovered that the mixer tap will only feed the shower and not the bath. The mechanism appears to be a chrime collar that I presume moves up and down to select either bath or shower. Are these things straightforward, i.e. what could the problem be and hiw to fix it. I don't want to replace the tap if I can help it because it woukd mean knocking out the airing cupboard wall. I can send a photo if the make/model makes any difference.