the contract for the space is binding save that it shall be deemed to be varied to allow for the necessary change in venue, dates or period of the Exhibition, stand size or location otherwise
A contract has to be read in its entirety. No one can give you accurate legal advice without seeing the whole contract and any other relevant information or emails that lead up to the signing of it - eg other negotations of terms etc.
With that note of caution, strictly speaking that wording makes the contract binding on both of you EXCEPT that gives the Exhibition contractor the legal entitlement to allow to vary it to make
NECESSARY (this is an important word)
changes in venue, dates or period of the Exhibition, stand size or location otherwise.
On the face of it they can do what they like. However, a contract also has various implied terms which include things like terms that would make the contract workable (called 'business efficacy'). An example of that would be say they said
our contract allows us to vary the period of the exhibition, so we are going to make it 30 seconds long
a court would say no one would intend that because it would mean that there was no workable exhibition - there is no business efficacy.
Terms can also be implied by custom and practice.
Was it possible to contract to exhibit for 1 or 2 days? Or did you have to exhibit for 3 days? If you could exhibit for 1 or 2 days, start with that and what the price was.
If you could ONLY contractually exhibit for the full run of the exhibition, I would argue that it was an express term that £full price was for 3 days. It is an implied term by reason of business efficacy and custom and practice that if the period of the exhibition was materially reduced, the £full price would be reduced pro rata.
The loss of one whole day is a substantial period of time amounting to one third of the contracted for period. Therefore by reason of custom and practice and business efficacy, you infer that the price per day is £full price divided by 3 and therefore the cost to you for 2days should be [one third full price] x 2 = £x.
You may get some push back saying it's not that simple as part of their charge is overheads/admin which applies whether it is one day or three, but it should open up negotiation.
Good luck.