Quite credible. There are numerous sources describing German miners ( sometimes mistakenly called Dutchmen ) being brought over to work in UK mines ( not just Coal ). They brought with them techniques for mining in hard rock, including early uses of explosives.

"The Talybont level itself was a remarkable achievement, being driven a distance of 200 fathoms or 1200 feet in four years; this equals about 6 feet a week. Although it is generally accepted that gunpowder was first introduced into mining about 1690, the real possibility that Thomas Bushell was using powder cannot be overlooked. Local mining historian Simon Hughes believes this is so and references the fact that Bushell was using some German miners and that blasting with powder originated in Saxony. There was a supply of powder available locally at Aberystwyth Castle; in fact a lot more than was necessary for strictly military use. When the castle was captured during the civil war it was blown up by the parliamentarian forces, indicating a large stock of powder already there rather than the parliamentarian forces having to bring it in from afar for the purpose. Otherwise they would have very likely just set fire to the place. Simon also draws attention to a later reference to the miner’s tools, where borers are mentioned. These could have been used for lime setting, but could also have been used to bore shot holes. If this is a correct interpretation, Bushell’s Level would be the first level to be driven in the country using gunpowder. To be precise, the very first recorded use of gunpowder in mining was in Slovakia in 1627, the first record of the use in England was in the Ecton copper mines, Staffordshire. A statement “Chemical Essays” written by Bishop R. Watson in 1781 mentions Prince Rupert bringing over German miners to work the Ecton Copper mines with gunpowder, this is further substantiated by writings in 1686 by a Dr Robert Plot mentioning “rocks being broken with gunpowder”. So there is a large body of evidence that has become generally accepted. This cannot in truth be overturned on the basis of what we have here; but readers can make up their own minds on this one. It is however worth mentioning in passing that Bushell was actually a good friend of Prince Rupert, something that would have made access to gunpowder easy if Bushell had wanted it."

The Lost Mine of Talybont - Roy Fellows

Relevant - top of page 3 http://www.aditnow.co.uk/documents//Hoosen-1747.PDF