another update:
The Declaration was accepted, the court told me the case would go back to the CPS who would decide what action to take.
I immediately telephoned Marstons and spoke to some stroppy operator, she said predictably they would continue with their action until told otherwise, to which I said your bailiffs will be wasting their time as I have just left court with a copy of the Stat. Dec. in my hand. After asking which court I attended she then implied I had gone to the wrong one because the fine was issued elsewhere, I explained it made no difference and that the issuing court told me the declaration can be made at any Magistrates (which, after having just successfully made it, proved to be correct).
I also contacted the court who issued the fine, who said they will request a copy of the declaration and send it to Marstons. I expect this to take time, and expect a visit from the bailiff regardless in the next few days, and I will drop him a copy of the declaration from an upstairs window!