Rising pressure in oil combi boiler (Worcester Heatslave 20/25) system

[tokenwelshman]

Hello
First post, hoping somebody can help me with some advice. I'm not particularly technical so bear with me.
I noticed last night that there was hot water trickling out of an outlet pipe on the outside wall by my Heatslave 20/25 boiler. Taking the front off the boiler I could see the system pressure was up to over 3 bars. I could hear a slight rush of water, as though the valve which controls the flow of water into the boiler had been opened (which it hadn't).
I vented the system pressure down to 1.5. Checked the inflow value, and although not possible to tighten it any further, I could hear and feel water flow in that pipe. So, the system is being continually fed a trickle of water.
Although the system seems to be dealing with the excess pressure by passing water out of the overflow pipe, I was not happy with the system being left at such high pressure overnight, so I turned the main stopcock off.
Fired everything back up this morning, identical symptoms, pressure rising and up to nearly 3. Hot water heating well and central heating kicking in now too.
I will shut the system down again once we leave the house. Not a very sustainable situation though.
Could this be as simple as the value controlling the water inflow has failed, or does this sound more serious? The boiler must be at least 16 years old given they stopped making the model in 1997. It has served us pretty well for the 6 years we've been here but I guess it isn't going to last forever.
Very grateful for any replies.
Thanks
Aaron

 

[tokenwelshman]

Bizarrely I think I have already resolved this issue and I'm not sure I really understand why.
Noticed the sound of rushing water at the far end of the house, where the outside tap is. Checked it out and the end of the hose had come off, water gushing out everywhere. Turned the outside tap off. Went back to the boiler - and I could no longer hear or feel water flowing through the pipe into the boiler. Vented the pressure back down to 1.5 ish and it seems to be steady now.
Not sure I understand why the flow of water past the boiler and out of the outside tap would cause water to keep passing into the boiler itself? Never mind.
Aaron