Hi Dai, good to have you on board. I've had a quick look for Joseph and J Parkers and it would seem there are quite a few. Any chance you know of a second name? There are quite a few British Army service records for Joseph/J Parkers on "Find My Past" as well as Boer War medal rolls. There are also a considerable number of J Parkers in the Boer War casualty rolls.
As a long shot I checked remaining WWI records (most having been destroyed in the Blitz) and found nothing for a Joseph Parker with Boer War service, born in 1881 in Aberdare. I did find the service record for a Joseph Parker from Aberdare, Glamorgan. His age differs though. On 4 September 1914 his age is given as 20 years 5 months indicating he was born in 1894 only. His father's name is given as Thomas and his address looks like 22 Abermont (?) Rd, Aberdare. This man served in the South Wales Borders, Cycle Corps, RE and the Royal West Kents. He was wounded in France on 7 June 1917 and in 1935 was living in Alberta, Canada. Perhaps this man is another relative?
Cheers,
Mark