[Spruce]

Its the battery I tell you! 

A fully charged leisure battery should read 12.7v or higher. If the Halford's battery was reading 12.3v then it was either flat or your voltmeter is faulty.
I would have asked them to get a fresh battery that was fully charged.

[Crystal-clear]

BINGO!
i drove to selco to find out they dont have anything what so ever related to batterys ! so i went to screw fix and found the same out was about to drive to maplin which i was avoiding asmy local maplin had no parking i purchased some new screws from srew fix 

and decided i will clean up those 10 year old clamps
raided my shed found some old connector cleaning wire mesh type stuff spent 1 hour cleaning them they came up pretty good can see the brass in them now then i screwed in the new clamps trimmed the wire more so now both screws were forcing them directly on the new cleaned clamps that now have a touch of brass on them
and case closed im getting good voltage
it isn't the battery honestly spruce i wish it bloody was lol the battery reads over 12.7v when nothing is happening mate and its running around 12.3 and 12.2 no lower then 12.1 and i tested it for a good 30 mins both pumps both switches once i stop drawing it returns up mate to 12.7 it wasn't doing this before it was going extremely low very fast 11.5 if i used both 11.2 or less amazingly my 6 year old numax is still working !!! and i shall look at a new battery but i knew when i saw the low voltage over night it couldnt have been you kinda see the battery die at work slowly

heres what happened in a nut shell :
so the chap added wires and i had my less wires properly touching those screws and if i remember the neutral wasn't actually screwed it was lodged in between as i must have had a voltage drop/power cut myself ages ago and i messed around with it
and it was working fine

but he tried to use those old screws to screw down the connectors and they didn't actually all reach properly the black was slightly touching and wasn't properly down

its fixed! this has now inspired me to not panic when it comes to the electrical side of things regarding our vans

i think i shall now extend my little 12 v whale sub pump to the battery and even add a switch so i no longer need to left those 25l barrels when i need extra water added to tank on the off day! i remember wanting to pay someone to do that but i think i have graduated now

[Crystal-clear]

oh and a very big thanks to everyone for each and every post some of them extremely detailed that i have saved them incase i have simialr issues next time i don't think i could have done it with out you thx again!