Fortunately they were booked on an Inter City which increasingly seems to be the only trains FGW▸ put a serious effort into keeping running.
The majority of First Great Western's business and profit is in the InterCity trains - regulated fares are higher on them (in pence per mile terms) than on the "West" services, and they travel faster too so the miles clock up quicker. Then you have the issue that you require more staff per seat on the shorter "West" services than you do on the InterCity-s so once again the business model is biased towards them; highly profitable, and the first place to put serious effort if you're loosing income because of problems.
I don't know what proportion of trains are cancelled over the current period - I do know that it was pre-planned to bustitute the TransWilts (so the services on that line will not show up in the stats as cancelled, even though they were!) with it being almost impossible to find out just what was running when.