So, in summary, it looks like you can send them two formal notices telling them to collect it within a "reasonable amount of time".
Send one letter now by recorded delivery, telling them to collect it within 28 days and that you will be charging £20 per week (or part thereof) storage, and goods will not be released until the full amount of storage costs has been paid. Remind them that they've had two week's grace so far, and storage is now being charged.
If it still hasn't gone after 28 days, send another telling them to collect with 5 working days or it's being sold to recoup the cost of storage, plus your costs of selling which may amount to more than the item is worth, then you will call in the bayliffs to recover the money owed....
That should get it shifted
