I need to run a length of 20mm MDPE pipe undergound for a garden tap.

To supply it, I will tee into the 15mm copper pipe supplying an existing outside tap. This supply contains a double check valve and can be isolated inside the house.

This leaves me with a few simple questions which hopefully you helpful chaps can answer:

1) are there any rules/regulations I need to follow for this type of installation?

2) how deep should the pipe be buried? (It will run under a patio so there's no chance of putting a spade through it!)

3) I understand that MDPE should not be left exposed to the elements, so should I make the change from copper to MDPE underground?

4) how are the 20mm MDPE to 15mm copper adapters like Screwfix item 16638 fitted? Are they simply push fit or are other parts required too? Also are they suitable for an underground joint?

5) last but not least, what do I need to do to protect it from freezing?

Thanks..