In addition to the medal roll databases, I’ve checked the actual medal rolls for him and found:
Roberts William; 23rd Foot; Military General Service Medal (1793-1814)
Clasps Salamanca & Vittoria.
Waterloo medal roll 1815
Wm Roberts; Rank: Private; Regiment: 23rd Regiment of Foot; Captain Harrison's Company No.3
There is only the one record in the medal rolls for a William Roberts who served in the 23rd Regiment of Foot (Royal Welsh Fusileers) and as you mentioned him being wounded, checking the WO97 Discharge papers at the National Archives, the only record for a William Roberts is:
William Roberts Born Gwyddelivan, Merioneth, served in 23rd Foot Regiment, Discharged aged 24; dates (served) 1811-1817.
The man in the rolls and the above must be one and the same.
The discharge certificate states:
Born Parish of Gwyddelivan, Town of Carwin, County of Merioneth
Enlisted for unlimited service at Wrexham, County of Denbigh[shire], 4th October 1811 aged 18
Discharged 24 June 1817 after 5yrs 265days service
Conduct very good
Reason for discharge given as “in consequence of Diseased Testicle & Receiving Gun shot Wound in the Left Leg at the Battle of Vittoria on 21st June 1813.
Item reference WO 97/431/150
So it would appear he recovered from the gsw enough to have been present at Waterloo but had more problems later.
Record is available online; from http://www.nationalarchives.gov.uk/catalogue/
enter the above reference in the small dialogue box upper LH side and click “Go to reference”