As a landlord can I suggest the following:
1: Talk to whoever lets the property for you. What has changed - why do they want a reduction? If they were happy to pay that at the start of the tenancy why alter it now? They should have negotiated that at the start.
2: Moving will cost them money - to sort out phones, Sky contracts, getting the deposit back, move furniture (I presume you rent to them unfurnished), getting the place cleaned can easily tot up to £600 without thinking about it. Plus, they'll need to pay for new tenancy agreement and credit checks etc, so that won't be cheap either.
3: I think you have been most kind offering to drop by 3%. You could say the rent stays the same as it is; but you won't put it up for another year. Why should you take a hit, because they don't want to pay a market rent?
4: You could talk to your landlord, but I would imagine he wouldn't be impressed by you asking to drop. The letting agents won't get so much either, so they won't be happy.
I think your tenants are taking the piss, good tenants or not. Yes, you're only losing £50.00 per month, but that can be the difference between eating and not sometimes. If you allow this once, what is to stop this happening again, when they feel like paying less? They are having a punt; say you'd rather not drop the rent, but can renegotiate to 3%, but that is as far as you will go, and see who blinks first.
ps: I have the DVDs of Blackpool to watch and Casanova as a reward for when I finish my GCSE marking. Sadly, ds (13) thinks I'm a very sad woman when I try to lick the TV every time his gorgeous Tennantness appears!!