Heating the whole shed will cost more than heating tanks individually. More losses equals more cost.
All electrical resistive heaters are 100% efficient whether tubular, oil filled, or fan type.
Thinking about this again they may not be in series (you would not be the first to use wrong terminology) but in parallel connected end to end. If this is the case doing what you suggest would work.
For a start that is not a transformer, it is an LED power supply, completely different.
If your LED strips are really meant to be in series each is meant to have 6 volts and connecting them in parallel at 24 volts could kill them. Don’t do it.
If you don’t want to wait for a replacement best...
If I was the seller I would tell you to go ahead and get an EICR.
The convention is that the buyer commissions and pays for any surveys he wants including electrics.
I would not trust a survey provided by the vendor for obvious reasons.