Unable to use onboard tank 2010 Sterling.

We've recently bought a 2010 Sterling Explorer which is identical to a Swift Conqueror 645 & the problem that I'm having is how to fill or use the onboard water tank?

The handbook states that there is a tap in the front of the caravan under the front drawers but I'm unable to find it? I've stripped the whole of the inside of soft furnishing & the chest of drawers to improve my view but still cannot find this tap. The caravan is a twin axle with the superb ALDI heating system & has a tank built in on the nearside floor, I've taken the top off to fill the tank manually & to add sterlilising fluid, the gauge now shows the tank to be full but I cannot do anything more with it, I cannot even find how to use the pump for the onboard tank.The previous owner cannot help as he bought the caravan brand new but never actually got around to using it! The onboard pump is at the front of the caravan under the nearside seat. I've had 3 vans in the past with onboard tanks & never had a problem like this. This is especially a problem now that winter is upon us & we'd like to continue using the caravan. can anyone help?