Recently acquired certificates:
Births.
Grenville James Gronow Dec. Qtr. 1902 Neath Vol.11a page 966
William Gronow March Qtr. 1905 Pontypridd Vol.11a page 734.
Marriages.
John Grennow March Qtr. 1873 Newport M. Vol.11a page 196.

I'm always on the look out for story's that involve our surname, growing up I was always being asked if I were German or Polish, because for some reason people always associated it with this variant rather than it's Welsh one. While searching the 1911 Census I came across a daughter whom I was previously not able to fit into a definite tree, looking forward I noticed that she appeared to marry twice. Upon investigation I came across this information which shows what extremes can happen especially in times of heightened security.
The names have been changed to protect the families from any embarrassment, and even the book does not name the Gronow concerned.
The following is taken from the book "In the Highest Degree Odious" by A.W.Brian Simpson.
"One case of error which is documentable received considerable publicity: it involved ********* who was British by birth and the Borough Surveyor of Guildford. He was detained on the 28th May as 'of hostile associations'; he was held in Brixton until 28th October spending up to twenty-three hours a day in solitary. The background was that 'his colleagues have informed against him and the local populace are demanding his internment.
"His wife is said to be of German extraction, and his intimate relations with a German girl have aroused the suspicions of the police" MI5 found that his wife was Welsh not German & that his association with aliens and visits to Germany were entirely innocent. Sadly his wife did indeed divorce him. In March 1941 he joined the RAF and became a Squadron Leader.

I remember a similar thing my father told me regarding his parents during WW1. The day after anti-German riots broke out in Birkenhead police tried to protect the first victim who had a butcher's shop at Watson Street but it was wrecked after police were overwhelmed by sheer numbers. Riots lasted for a couple of days and pubs were closed. Some luckless victims were not German related. Fried fish dealer Thomas Lincoln, of Price Street, had his shop smashed up by a mob made up mostly of women and young boys. They looted everything after someone said he had sold a fish to a German!