1. Under what circumstances do you use a SINGLE non-return valve, and
when do you use a DOUBLE non-return valve ? What advantages/disadvantages
does one have over the other ?

Single valves are cheaper and drop less pressure. Double valves are more
reliable and required if water byelaws require a check valve, such as before
a shower, or garden tap.

2. Whats the difference, apart from a few pennies, between an ISOLATING
valve and a SERVICE valve ?

They are both the same, except a service valve has a tap connector one end,
whilst an isolating valve has compressions both ends.

Q1 is for a pumped hot-water loop, and the valve is to ensure water goes
the right way round the system ie preventing the fresh cold water entering
the loop from going the shortest way round the loop and bypassing the
thermal-store heater.

I'd always use doubles, but a single would work here, provided the schematic
of the system shows that the valve isn't being used to prevent contamination
of the supply.

Christian.