Consumer unit tripping

Long story and probably even longer responses, but here goes...
(am currently on second year of 2330, so am not qualified, but am "sort of" competent!! I also have access to qualifed sparks when necessary, so please no lectures on Part P etc)

Friday afternoon the electric hob tripped the house. I have an MK board with 100mA earth leakage circuit breaker for 7-ways. Oven had been on for a while, but ceramic hob was the breaker. Run on 6mm from 30A breaker(?) thought they were 32 as breakers and 30 as fuses????
Later, having isolated that circuit the electric shower goes, and its on a 32A breaker with 6mm cable. Have since found that previous occupier had cut into this supply and run the power and lights to the loft from it, as well as the switched fused spur to the boiler!!! I have since cut this from the circuit and run the loft and boiler from a fused spur previously used for the immersion heater (that was removed due to a fire that started in the airing cupboard!) YES you can see a pattern emerging here...
On further investigation have found that the only 'safe' circuits seem to be the 5A lighting circuits as the others are all overloaded in one way or another.
I have had cavity wall insulation installed and the cabling runs thru this to the various floors, thus compounding a seriously overloaded circuit.
I am now waiting for a new board and respective MCB's RCD's, with no cooker/hob circuit or electric shower circuit.
THEN....this morning....just turning on the hoover has tripped the house again.
What am I missing here? Is the main switch tripping easier or do I have a leak to earth?
Any advice would be appreciated

Al
(PS sorry about the username, but its a long standing one with an even longer story!)

What earthing arrangements do you have? TNCS , TNS, TT?

If TT (as it sounds like it might be) then I'm hopeing the new board you have ordered (?) is a 100S/30 split?

circuit sizes were harmonised a few years back from 5/15/30 to 6/16/32 , older breakers will follow the old scheme.

And yes sounds like neutral to earth fault that needs enough load passing through the RCD before it shunts enough to earth to trip it

Its definitely a TN-S system...ex-coucil house about 1960's construction. But think the earthing is 10mm not 16mm so looks as though that may need a look at too. Will double check that though.
Dont know when the re-wiring was done but think it was some time ago, and the previous guy (a housing inspector for the council!!??) was a bodge it merchant. The old black conduit is still in evidence under the boards and poking thru some walls with the old style cabling cut off inside.
What may be the cause of a neutral to earth fault?
Where do I start...other than by splitting the obviously overloaded circuits that is?

Hi Diva,
Sounds like you need a spark with his test gear.........

14th

On further investigation I have found a third 6mm cable running out of the isolation switch for the shower. It was just wrapped in insulating tape so I put a block on the end to finish it properly. Any ideas what this was for? There is the supply to the pull switch looping out to the isolating swithc and then on to the shower. This third wire is coming out of the isolator....

Breakers Can be 30A too, as well as fuses. Could be type 1 or 2.