Outbuilding Wiring - Power Tripping

Hi
My partners ex had a shed/workshop that looks like it has been professionally wired.
The feed comes in and then to four double sockets in series.
Between the first and second double socket there is a spur that comes off and feeds a light switch and then a fused switch.
The shed runs a small chest freezer,odd power tool and mower when required

My partner bought a water feature and her father installed an outside single socket with built in mechanical timer - the water feature is plugged into this.
To the socket he has wired a 13amp plug and this is plugged into a double 13amp RCD front plate/box which he fitted by replacing the fused switch.

Everything was running okay for a few days and the water feature was on/off as timed.
Now when the off is activated the shed RCD trips and so does the MCB.

Can anyone help please and is it the RCD that is tripping the mains power and could this just be simply replaced by a double socket.Does the shed require the fused switch being reinstalled or another added.

Thank you for your help

You really need to be getting a spark in for those jobs you cant really just spur off and anywhere you like i no my advise hasnt helped but these things are regualted for a reason electricians go on regular coarses to be upto date (which saves lives) please just get a sparky in he could be next to nothing ££ to sort it out (if you get 1 that you can email pictures too he might even give you an idea on price before he comes)

A spark may come out if he is just going in the garden, not the house.

smells of a fault with the water feature.
Can you just disconnect the lead to that leaving the timer socket etc in place.

if it still trips then (it’s been raining) maybe water is inside the timer socket. Power off and undo the front plate. Is it wet inside?

The 4 double sockets in the shed won’t be in series (I hope). Replacing a fused spur with an RCD is stupid. There is no longer a fuse for the light.
Is the feed to the outbuilding already RCD protected? Definitely need a spark to sort it out properly.

I'd be suspicious of the water feature as well as of the outside socket 'fitted' by father in law ish. Supplied by a bit of flex. Has the 'installer' fitted all the gaskets, sealing washers etc required to maintain the outside sockets' watertightness? Why didn't he just use a plug-in RCD in one of the 4 existing 2 gang sockets (that are NOT wired in series).
Running any outdoor appliance without an RCD would be foolish (you say you run the mower from the shed sockets occasionally- as above is the outbuilding feed already protected by an RCD?)
First job- return the shed wiring to the state it was in before partners father started messing about (so put the FCU back in the lighting circuit where it belongs and get rid of the outside socket)
Second job- find out if there is an RCD protecting the shed supply
Third job depends on what you find after Jobs 1 and 2

Thank you all for your replies it is very appreciated.
The water feature has a moulded plug connection which is plugged into an IP66 on the outside of the shed
From the IP66 is a small length of flex that then comes into the shed and has a plug connected,this was then plugged into the RCD.
I would guess he changed the fused switch for a double RCD socket as it was close to the IP66 outside and for the mower to be plugged into.
We can't ask him at the moment as he is in the services and away on TOD.
There are no other RCD's in the shed and it is connected to the main consumer board.
All cabling from the house to the shed is buried along the fence line but it looks like it is armed flex then going through pipework.

Taylortwocities - inside the IP66 it is bone dry

When I say series I meant they are series in line and not the way they are wired (I wouldn't know that and sorry for the confusion).

I have removed the RCD and box and reinstated the original fuse switch.

The water feature is still plugged into the IP66 and where it comes into the shed I have now plugged it into an extension cable,which itself is plugged into one of the sockets.
It has been running for over two hours now with lots of on/offs and no tripping.

So going from what oldbutnotdead has said can I exchange one of the socket faceplates for the RCD one and then run a longer plugged flex from the RCD socket to the IP66?

Thank you.