I suspect that the advertisment for the 300 men at Radcliffe was the colliery's first expansion after the shaft was sunk. First coal was 1837 but the actual First Pit shaft wasn't sunk until November 1839.
http://www.dmm.org.uk/colliery/r020.htm
Additionally, although possibly functional before this date in some capacity (and the waggonway from the colliery to the port was probably been laid around 1840), Warkworth Harbour wasn't completed until 1844. Though, I suspect "completed" would mean all the fineries.
Although, after these times, Durham County Council have a great history website which includes a great document by Derrik Scott on the mechanics and politics of the 1844 strike which gives an indication of how unsettled relationships were with the mine owners and coal workers.
http://www.durhamintime.org.uk/Durha...eat_Strike.pdf
It also has a great snippet about Radcliffe Colliery which I'm assuming must have been after the strike had started:
"Dirty tricks were employed by the coal owners and their agents in order to entice workers from all over the country to work in the mines of the north‐east of England. An overman from Marley Hill Colliery, Co. Durham trying to recruit men in Staffordshire reported that the strike in Northumberland and Durham was settled, but Marley Hill, being a new colliery, needed men. The agents of Radcliffe Colliery in the north brought 32 miners from Cornwall to replace their workers with promises of 4s per day. When they received their pay of 3s or 2s 6d per day they struck for two days until they received assurances that the 4s would be paid. (This cost the owners money because the Cornish men were poor coal hewers – not so good as the north‐east pitmen). When the owners offered the Cornish men 4d per tub (they only filled about four tubs per day) they absconded. A reward of £50 was offered for their capture but some escaped (possibly to go home). Some were captured and tried at Alnwick but were acquitted."
Derrik Scott (2011) Great Strike of the Northumberland and Durham Coalfield in 1844, Durham County Council.