Doh - you gave the details above, sorry for wasting time ๐
Ah - this looks to be the battery you are using, which is designed 'specifically to [fit] under the seat base area of the Sevel X250/X290 based chassis. ie. the front seats of the Fiat Ducato, Peugeot Boxer or Citroen Relay chassis base that is favoured by many British and European motorhome / campervan manufacturers.' (from: https://www.rhinoinstalls.co.uk/KS-Energy-12V-200AH-Seat-Base-lithium-battery-with-Bluetooth-BMS).
I will measure up my under-seat area but I suspect that, even with the insulation removed, a normal leisure battery won't fit under the drivers seat of a 2005 Alphard. I would need 22cm room.