Vendor not moving out by contract date - advice?

All,
The story so far is that we signed contracts for the purchase of our house on Dec 1st. The closure date was March 31st. About a month later the vendors still had not signed them and when they went to do so, they changed the date to end of June!! We were pretty annoyed but our choices were either to put up with the new date or pull out and we didn't want to do that so we had to move out of our apt and wait 3 additional months to get into our new home.
We were due to move in next week, deliveries ordered, vans booked etc when 2 days ago our solicitor got a fax to say the vendors now wouldn't be ready to move for a further 3 weeks after the contract date. We are so upset and feel like we're being taken for mugs at this stage (the house went sale agreed last Oct!)
Is there anything we can do? Can they get away with doing this? The biggest problem is after all this I feel we can't trust them and wonder what stunt they'll pull next. They've also decided that they don't agree with the contents valuation. Now surely this is just some sort of stupid game, delay tactic, trying to create more work on our side or something (btw, the valuation was provided by the estate agent it's not like we made it up!) Anyway,surely the valuation of contents should be irrelevant to the vendors, it has no bearing on anything other than the stamp duty payable which is between us and the Revenue and our solicitor and noone else.
Apparently we can issue a notice of completion which gives them 28days to move out and then sue them!! Is there anything else we can do in the meantime? Have we any entitlements in this situation?
Do buyers have any power or are we just going to be walked over time and time again....
Any advice would be appreciated,even evidence that we're not the only ones this has ever happened to would be good.
Thanks
Shakespeare

You should issue the 28 day notice as soon as possible and not wait to ensure that they are not moving. This will just give them more time.
As to the contents valuation. You have both signed the contract. What does it say. You must stick to the contract

The 21 days is for new properties only, you can issue 10 days on a second hand property (would have thought your solicitor would know this!)

Sorry to contradict, but there is no special "10 day" rule on the purchase of a second hand house. It can happen from time to time that solicitors will insert in the contract a period other than the standard 28 days - but this is not normal (and in any event is more common in new houses)

I'm no expert so wouldn't be sure (should have put that in original post) but have heard of the 10 days being enforced on 2 seperate occassions, so was under impression that is was a clause...in fact it was a estate agent who told me that 10 days could be enforced on second hand properties - should have known better!

Can you charge them occupational rent from the date they were supposed to be out?

Not all sellers are the same. Some do put their hands in their pockets and rent for the period while waiting for their own new house to be ready. I did. It sounds like the sellers in your case need some legal encouragement to do the same.

Without the history, I would have approached it using gentle persuasion, but with the history you have outlined, I would have the completion notice issued and leave it to them to come back with a possible compromise if they are so inclined.