"Martin Rich" wrote in message
...

Somewhere I also remember reading that the Irish Sea is much deeper
than the English Channel, which makes tunnelling more difficult than
the tunnel length would suggest, but I haven't been able to verify
that

From a wander around GOOGLE I find......

The Irish Sea is a semi enclosed shelf sea bordered by the island of
Ireland, Scotland England and Wales. The depth in the western Irish Sea is
characterised by a channel of greater than 80m depth that runs from St.
George's Channel in the south to a maximum depth of 275m in the North
Channel.

also...

The English Channel has a maximum depth of 100 m at the western mouth (5deg
W) shallowing to 40 m in the central Dover Strait


Which does confirm your memory.

KW