The fact that it's an alteration made to an asset owned by somebody else doesn't prevent it from being capital. For example, alterations to a leased building are capital, even though it is the landlord that benefits in the long run.
I think that the key question is the length of the hire agreement. If there's more than one year left to run, then there is a case for treating the cost as capital.
I'm guessing that the OP doesn't want to take the hit of £5,000 against current year profits and is trying to spread over a period