Has anyone encountered the dreaded electric parking brake failure. I managed to manually release it for now. Will be booking it in local garage. Is it a big job. How long to fix and estimate in terms of cost.

possibly just needs clean and adjust. Have a look at the brake bible in my sig.  Bodsys Brake Bible
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It is had jammed on, and you had to pull the emergency release, there is a good chance the mechanism inside will be jammed.
As Bodsy says, try a strip and clean firstly, but if that does not cure it, you will probably need to try this: EPB Unjam + Relatch

Just had mine done
New electric actuator, new discs, new pads new shoes and new pad wear sensor.
parts came to about £600.00
Labour(friend of mine) wanted to charge me £ 120.00 but gave him £ 200.00 I still think that was cheap for amount of work.
I just thought if bits coming off replace with new. 

Supporting the London Air Ambulance with my Brothers

Mine was sorted by the RAC chap re setting it with his computer, worked ok ever since TDV8 2007 window tints, side steps.