Flying LHR-CPT in CW in October.

We have a inflatable tent to return to some friends out there which is 78 x 58 x 53 cm and circa 40kg!

I rang the gold line and they say its a flat fee of £65 for oversize baggage, but the website says nothing over 32kg.

Any one have any experience of whether we can take this with us, and if so is the agent correct about the flat fee (most airlines charge per Kg over a set weight...) ?

Many thanks