Boeriemore

Thank you Superdiy, As a relative layman, I feel that the most likely culprit would be item 3.

Thank you for that helpful table. So I wasn't far out with my calculation.

I must ask if I was to restrict the temperature control on the unit to a low setting would it draw less amps thereby enabling the 30 amp breaker to cope?
I cannot simply install a 40 amp breaker as the  breaker is located in the house and the unit is in an outbuilding so the cable from the house must be buried under
paving, then into the outbuilding to its own  25 amp breaker. 

I will do the dirty work first, drilling larger holes through the walls to fit the larger cable I will have to buy, also a two pole switch.
Basically I will get an electrician to came and inspect and correct any error I have made.