Meter Tails.

You don't say why you are replacing the tails

: Is it just a CU change? It's unlikely they will ever overload in a domestic house. It's rare that you couldn't replace them but it might be really involved /expensive if you had to re-route. If you fault it and the customer says that's too expensive, you can't make them, althugh you might consider not changing the board if you don't want to put your name to it.. There doesn't seem a lot of point changing part of them up to a henley block. If there are any issues its likely to be  with the bits you cant see rather than the bit you can. If you advise and record it then you've covered yourself. also you could work out the maximum demand ( unlikely to be more than 80A I'd have thought if the heaters are on their own supply) Measure Ze, do a calculation to prove the fault current would be sufficient- yes they should be 25mm but its unlikely to be a problem. If youre replacing an old board you can argue you are leaving the job safer. But if you are trying to do a full rewire then I'd suggest that you need to think about a way to replace them completely. There's always a way.

You don't say why you are replacing the tails

: Is it just a CU change? It's unlikely they will ever overload in a domestic house. It's rare that you couldn't replace them but it might be really involved /expensive if you had to re-route. If you fault it and the customer says that's too expensive, you can't make them, althugh you might consider not changing the board if you don't want to put your name to it.. There doesn't seem a lot of point changing part of them up to a henley block. If there are any issues its likely to be  with the bits you cant see rather than the bit you can. If you advise and record it then you've covered yourself. also you could work out the maximum demand ( unlikely to be more than 80A I'd have thought if the heaters are on their own supply) Measure Ze, do a calculation to prove the fault current would be sufficient- yes they should be 25mm but its unlikely to be a problem. If youre replacing an old board you can argue you are leaving the job safer. But if you are trying to do a full rewire then I'd suggest that you need to think about a way to replace them completely. There's always a way.