Found this on Trading Standards:
Installing it nonetheless could be seen as accepting the product in the state it arived, but you are right to make others aware of this deception.
Still think if you knew upfront the longest length on the board described as being 400 - 2100mm would be 950mm maximum you would not have bought it.The Consumer Protection from Unfair Trading Regulations 2008 prohibit traders from misleading consumers about a product by giving false or deceptive information about a number of matters. To breach the majority of the Regulations, the misleading action or omission must cause, or be likely to cause, the average consumer to make a different transactional decision (eg make a purchase which he would not otherwise have made).
Installing it nonetheless could be seen as accepting the product in the state it arived, but you are right to make others aware of this deception.