....There's nothing really that can go wrong with the valve block....
Just a bit of thinking out loud here....
I believe the pump fitted to the Laguna is the ATE MK60.
This pump was also fitted to a lot of other vehicles, VW used it on their vehicles. VW had a lot of bad press because of failing ABS pumps, take a look on the web.
The problem was apparently down to the internal Brake Pressure Sensor that is housed in the valve block of the pump.
Some VW's Fitted with the ATE MK60 pump had an external Brake Pressure Sensor that was fitted to the master cylinder, so either the pump was available without the built in sensor or they just used an external one anyway. The cars with the external sensor didn't seem to have the ABS problems.
Onto the Laguna....looking at my master cylinder there is no pressure sensor on it, so I can only assume that it is internal in the valve block of the ABS pump.
Looking at pictures of mk2 Laguna master cylinders (on ebay) I can see some that have the pressure sensor and some that don't.
Given the VW info I assume that if the master cylinder has a pressure sensor that the ABS pump either doesn't have the internal Brake Pressure Sensor or if it does the external sensor is used just like on the VW's.
What's the point of this post....
well I've been assuming that the fault on my original ABS pump was more than likely down to the ABS ECU being dodgy, I assumed this as I know the motor of the pump works ok (tested)
I figured there was nothing that could really go wrong with the valve block, so that only left the ECU. But as you can see above this doesn't seem to be the case, maybe the fault is nothing to do with the ECU and it could be the internal Brake Pressure Sensor in the valve block that caused the problem, that is of course if there is a pressure sensor in the ATE MK60 pump fitted to the Laguna.
I hope you follow all of this & I apologise if I've confused you
All I know for sure is that fitting a 2nd hand pump cured the problem and my ABS works fine now.