If he was a Territorial in the Welch Regiment for 5y, there is a possibility that he served after the 1920-ish cut-off date. After that date, ordinary
soldiers' papers are supposed to be still with the Ministry of Defence. You could, therefore, try to see if they have his papers - there are all sorts of restrictions and it's possibly best if you get the Next of Kin to apply. If
his Territorial papers are with the MoD, then his WW1 papers may also be in the same bundle, or at least there may be a reference to previous service as .....
However, I'm just a little confused about whether the MoD has
service records for Territorials who weren't mobilised during WW1 or WW2. http://www.nationalarchives.gov.uk/help-with-your-research/research-guides/volunteers-territorials/
is the TNA Guide on the topic, while another guide is http://www.whodoyouthinkyouaremagazine.com/tutorials/military/territorial-army-ancestors
- make sure you read the "next" pages as well as the one off that URL.
I have a feeling that the MoD may not
have Territorial Service Records if someone served only between the wars. Maybe the Welch Regiment's museum can advise what they have....