No power to circuits - RCD not tripping

[kickaha]

Domestic CU garage unit. Power is incoming at 237v and registers Live on input and output of RCD. It supplies 3 MCB's on a busbar feed all of which show Live on output as well. Earth is TT and Neutral is on a common bar. Its been fine for 10 years but all of a sudden the lights and ring have just not worked (despite Live output on all of the MCB's). The I changed the RCD as the test button on the old one failed to trip, but the new RCD fails to trip as well.
I used to be an electrician some years ago and only have basic test eqipment now. I suspect a Neutral problem but am a bit flummoxed!

the only odd reading I get is RCD off. L - E at incoming is 237v N - E 37v
RCD on L - E at incoming is 237v N - E 0v
RCD on L - E at outgoing is 237v N - E 0v

I've forgotton a lot of theory so any hints and suggestions as to how I could proceed would be very much appreciated as I can't work on my old Land
Rover!

thanks, Clive

 

[Welchyboy]

To be honest out of all the the rcd faults ive had to sort out I think I've only ever had one or two faulty rcds. Even the older ones I have found to be highly reliable

 

[kickaha]

Thanks for all help offered. The cables were buried in the grass and an old metal junction box had a leak from one of the armoured cable glands. The Neutral connector had melted away, hence to huge resistance reading. New Wiska box and cables tacked to the concrete wall should keep it safe. Drove me mad as I only thought the supply cable went straight to the workshop. Can work on my Land Rover now! Many thanks chaps.
Clive