It would help if we knew what was on the other end of the wires so as to know what they were doing. However, it wouldn't be too difficult to replicate exactly what the C27 is doing now with a single channel Hive. First of all as per
@ianmcd
Then if as you say:
If a permanent live has to be maintained here, then putting the blue wire with the brown sleeve on in the L terminal would replicate the hot water being switched 'on' 24/7. I suspect it might be the way the installer has provided a permanent live to the boiler. But like I said without knowing what's on the other end of it...