What is this?

[Olly75]

Would anyone care to hazard a guess as to what this is and when it's used (tje second part is probably self explanatory once I know what it is!) Discovered earlier in my laz.

[CSY Man]

Mushroom anchor..
Use it for a mooring if you bury it deep enough.

[Olly75]

That's great, thank you.

[captain465]

It is a mushroom anchor...........but, judging from the photo, not big enough to use as a mooring for for your "about 35" ft boat. It was very likely used as an anchor for a dinghy, or as a "kellet", to aid in the anchoring of your boat.
A kellet is used to effectively increase the scope of your anchor rode by keeping the rode more parallel with the bottom of the seabed.

[StuM]

Yep, mushroom anchor. Good for holding in soft mud, but can be a real b*gger to retrieve. They don't break out easily. As CSY Man says, can be used for a permanent mooring (if it's large enough and it can bury itself deep enough in the soft stuff)

[zeehag]

small ones make great dinghy anchors for beach landings