Halfords are pretty poor by the way the deal with repairs.
Well, the one I work at is.
We book them in, and then they pretty much do them whenever, it's not always the staffs fault that the new parts don't come in whatever.
When a new componant is needed, we have to ring head office, sort it out with them first, before we ring the companies or whetever, and then get the componants.
But some companies are slower then others, so the customers may have to wait a long while before the repair is done.
Anyhoo, if you were having your hoses cutting down, surely they would have to take the magura apart, and re-bleed it.
And because Halfords have been told/trained etc to use bleed kits, and Magurs Olive, that's what they'll do.
And to keep the customer happy, they'll do the best job they can.And to do this with you, they would have to dry out the magura etc, To keep it in fully working.
You could just ask them to cut the hose down, put it all back together without a bleed, so you could do it yourself.
For the future, learn how to repair your bike yourself, It doesn't cost anything (labour wise) and will take half the time.