I would leave it for a few months and decide that the company had abandoned the money, thus making the Theft Act not applicable
In my eyes they are a huge corporation and don't care whether they get their money back at all, therefore, Theft Act sidestepped, wooohooo
Edit:
Some helpful law actually relating to the situation:
s. 5(4) provides that:
“Where a person gets property by another's mistake, and is under an obligation to make restoration (in whole or in part) of the property or it's proceeds or of the value thereof, then to the extent of that obligation the property or proceeds shall be regarded (as against him) as belonging to the person entitled to restoration and an intention not to make restoration shall be regarded accordingly as an intention to deprive a person of the property or proceeds.”
s. 5(4) covers the situation in which the owner transfers property to the defendant under a mistake which is not sufficient to prevent ownership of the property passing to the defendant. Although ownership has passed, the property is treated as still belonging to the original owner. Thus, if the defendant realises that there has been a mistake and fails to return the property, intending to permanently deprive, he can be liable for theft.
The obligation to return the property must be a legal one not a moral or social one, see Gilks 1971 in which a mistake which led to over payment on a gambling win was held not to fall within s. 5(4) as gambling debts are not legally enforceable.
The person will only be guilty of theft if they are aware of the mistake, being dishonest and intend to permanently deprive the other of it - Attorney Generals Reference.