BLTouch doing poor job of levelling

I’d appreciate any advice on this problem. I have and Ender 3 Pro with a glass bed. I have been using Marlin 2 with mesh bed levelling and it has been working really well. I have now installed a BLTouch version 3.1. I have used the TH3D Marlin V2.0.5.3 set to probe a 3 by 3 grid (same size I used with mesh bed levelling. My glass bed has a dip in the centre. When using mesh bed levelling this was found to be around 0.05 mm lower than the rest of the bed. However, the mesh bed levelling worked well and I got great first layers. Now with the BLTouch, I’ve set up my probe Z-offset, done with the bed and nozzle at full printing temperature. I’ve got a test print of 9 small squares and have sliced it in Cura at 0.12 mm. It’s just one layer. No matter how much I fiddle/tune the Z-probe offset, I am getting poor results. The square at the front left of the bed is a complete load of spaghetti. The others are better but not perfect. It’s as if the bed levelling process is not working properly. But it all looks like it’s doing the right thing. Is there any way I can verify the measurements the probe has made? Is there anything else I should check? I’m really at a loss. I’ve also manually levelled the bed to make sure the four corners are level with each other. I’m completely stumped!