Generally, the doctrine of caveat emptor (i.e. 'buyer beware') applies, and as you had time to inspect the house and did not discover it or raise such issue before, you would be precluded from doing so now. However the fact that they seem to have tried to cover it up to avoid you initially detecting it is relevant. You could argue that this deceitful conduct is tantamount to a fraudulent representation i.e. they (mis)represented the property to be free of mould by putting up the wood and not telling you about it, and the purpose of that was clearly to deceive you such that you would not find the defect.
You would have 2 years to sue from the date you discovered, or ought to have discovered the mould, as per the Limitations Act.
If you are seeking to pursue this case, it would be wise to hire a lawyer as this will likely be quite a litigious case, given the argument that the previous owner will put forward (i.e. they'll claim you accepted the house in the state that you looked it over).
Hopefully that is of assistance.
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