Earth cable size to bridge a copper-plastic-copper water pipe

Without examining the situation more thoroughly, it does no harm to use two safety earth clamps and a 10mm squared bonding conductor. You can buy this from a wholesaler in short lengths. You may be able to get away with 4mm (supplementary bonding) but you can't get into trouble by using a 10mm bonding conductor to ensure really good earth continuity across the plastic section. At short lengths, the price differential is small so use the bigger one. This is a safe bet, because I have no idea where this plastic pipe is in relation to the main bonding conductor which should be connected within a short distance of the stop cock and then connected back to the main earth bar near the fuse board. In addition, if you haven't got one, an RCD fed fuse board should really be installed in in order to provide protection against future electric shock. They disconnect the supply within a very short space of time if an earth fault occurs -particularly important in wet areas such as bathrooms and outdoor sockets. You'd have to call in a qualified electrician as this piece of kit installtionrequires notification to the local council building control. Alternatively to the above, you could call in an electrician to assess the bonding condition of all your pipework and assess whether supplementary bonding is required. He or she could then talk to you about RCD protection for safety if you haven't got one already. Isn't it complicated?

It's a good question, and the answer isn't straightforward: Without getting a registered, competent electrician to inspect the electrical installation and take test measurements - it's going to be very difficult to make any recommendations: The answer could be anything from none at all, to 4 sq mm. You need to consider if there's RCD protection for all circuits, whether there's existing main and supplementary bonding in place, and what the resistance is from extraneous metalwork (e.g. washbashin taps, radiators) back to the main earth point and between the taps and other extraneous metalwork.