Nespresso Lattissima One Stuck in Descaling Mode

Hi all,

I've recently bought a Delonghi / Nespresso Lattissima One (EN500.W) on ebay which was sold as faulty. I bought it as a project to try and fix it. Basically its stuck in descaling mode. The pump won't run when the machine is switched on. All the machine does is blink - all of the lights on the top blink rapidly. Pressing any of the three buttons doesn't do anything, no matter how long they are pressed for or in any combination. I've already bought and fitted a replacement pump for it, thinking it was the pump that was faulty. Sadly not; the machine handling exactly the same thing. I'm now at a loss as to what the issue might be. I'm thinking maybe a faulty flowmeter? But I really need some advice from someone with more experience at fixing coffee machines than me to point me in the right direction.