Thanks all.
I'm not sure why I thought Bath was next to the sea, maybe the watery name! Obviously something was there.
Of course, it is entirely possible he moved elsewhere to work on PLUTO if he was involved in the physical side of things, I haven't yet found out where the ship side of things was based. The war was six years long after all! In an article in Norwood News in 1948 it says that Walter and his wife had lived at their address in Upper Norwood for sixteen years. Bath is a significant place in my family- after speaking to mum, turns out my great grandfather's third son also worked at the Bath admiralty and so did my grandmother- that's how my grandmother ended up meeting my grandfather (and the rest, as they say, is history).
If he was involved with the planning rather than the manual labour that is certainly an additional piece of the puzzle for his wartime career, but again I don't know if there is any way of actually finding out solid information on what he did. A lot of the time, including the 1939 Register and his son's marriage in 1948, Walter is down as a "civil servant" but other documents specify his ship work.
Shipwright apprentice Govt Dockyards (1911)
Shipwright HM Dockyard (1917 and 1919)
Assistant Naval Constructor- admiralty, retired (1953)
Assistant Naval Constructor (retired) (1978)