I was looking into leasing a new combo (1.3) and after some research on the "what van" website it said that the payload was only 525kg

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Maybe I'm wrong but, they must have a higher payload option, 525 is pants! Could barely get a 250 tank and all your bits to be within the figures.
I used to have a berlingo (600), that had a 250 flat tank fitted along with omnitrolley ladders hoses poles and me it used to be very low and probably border line on the weight.