Restocking fees?

Hi.
We ordered a shower enclosure +kit (costing £1200) from MKM, Hull through our plumber who said he had a trade account with them. It was delivered within 3 days. However, the day it arrived, our plumber, who had by then ripped out our exisiting shower, realised that the shower that was ordered will not fit into our existing plumbing setup & would require extensive replumbing. So he called up MKM (within 2 hrs of the items being delivered) and asked if they'd take it back as it didn't fit. The packages had not been opened. MKM said they'll take it back but are charging 20% "restocking fee" - that's over £200!!! Is this a reasonable charge for returning items within a week, unopened?
They did offer to waive the charge if we bought a replacement from them but we couldn't find anything suitable. Do we have any choice other than to either pay up this extortionate amount or buy some other unnecessary & expensive item from them? Please advise.

Hi Tony.
Thanks for your reply. When we picked the item from the brochure, neither us nor the plumber knew about what lay underneath the existing shower (was a rather messy job by whoever put that one in!). The plumber wants to pass on the cost to us because we're not willing to buy the replacement from the same company (in which case they'd waive the fee) & would rather go with better options (in our point of view) from a different shop & from whom we're buying directly & not through him.