2003 megane Auto gearbox jumping Gear

[3jwmegane]

2003 Megane 2.0 VVT Automatic ( 42K Miles) (Which is much loved) is jumping out of 4th Gear into 3rd. 'Check Gearbox' and 'Service warning lights illuminating.
The car continues to drive in 3rd but will not change back up to 4th. Stopping and restarting the engine clears the problem until it decides to do it again.
There is no pattern to this fault. It does it at random, hot, cold, slow, fast.
I've been advised a fault with the Pressure Regulation has been diagnosed and the Hydraulic Distributor has to be replaced. £800.00 + VAT.
Does anyone have any advice how to overcome this problem without having to spend such a sum? Please.
Regards BJW