Expansion Bottle Leaking

On my recently purchased Disco 2 td5, amongst its other issues, is the water leak out of the expansion bottle. If this if filled, when I stop after driving far enough to get it up to temperature, it seems to deposit the contents of the expansion bottle over the road, or drive. The water is clear, and there is no evidence of it escaping anywhere else, or any emulsion in the oil filler cap. My mate suspected some heat sink issue which caused the pressure to build up once warm. Would this suggest a cap maybe or a crack. I can't obviously see one but I wondered if someone had had similar issue.
I don't reckon on head gasket (hopefully) but I like it dripping water less than I do a small oil or diesel drip.

Put a new tank cap and tighten it well to rule it out first... it must seal well and manage the pressure through it's built in pressure controll valve as to keep it at 1.4 bar steady all the time or you might have all kind of troubles

Hopefully it is no more than that. Will get one of eBay and prob a new cap too.

Turned out to be just the bottle where the plastic foot locates, so we seem water tight now.
A bit odd though, pulled the bonnet this morning and the top radiator hose had suck itself in. Seemed alright after I released pressure on the expansion bottle cap. Air lock?

have you raised the tank above the top hose while filling and bled it well?