If you have fitted an Indirect cylinder as you say ?
Then it must be faulty or it could be a Primatic ???
Does it say Primatic or single feed cylinder ?
Water will find it's own level so you must have something higher than that vent pipe pushing it out if as you say the pump and fire are not on ?
Is there another tank higher ?
Try one of these
http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=100122&ts=79320&id=23297
Plus one of these
http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=100122&ts=79366&id=14890
OR this
http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=100122&ts=79540&id=17647
And don't forget this...