Yes, it looks to be an issue with lat/long. Seems to work fine, but only if both the centre and the markers are grid references
[osmap centre="SJ6355257675" zoom="4" markers="53.115000,-2.546000;RACUNDRA at Venetian Marina"]
marker is at correct place, but still centres on Berwick.
[osmap centre="SJ6355257675" zoom="4" markers="SJ6355257675;RACUNDRA at Venetian Marina"]
works.
I noticed there is some javascript code to attempt to centre the map, if no centre parameter is supplied, and it obviously does work for GPX. I only need grid references, so I’ll not dig further into this.