The striker should be eligible to feature in the Robins' League Two clash against Colchester United on New Year's Day.
Jamie - who celebrates his 20th birthday today - missed the first three months of the season with a dislocated shoulder but has showed signs of late of returning to the form that earned him a first-team call in March 2003 with four goals in his last five starts for the reserves.
A prolific goalscorer at reserve and Academy levels, the Enfield-born player has been selected by England at Under-18 and Under-19 levels.
Meanshile, Jamie's reserve team-match Paul O'Donoghue has joined Hornchurch FC on a month's loan.
The Urchins are currently sixth in the Nationwide South.
Centre-back Paul has played well and skippered the reserve side on occasions this season but has had less opportunities of late with the return of Dean Richards.
Jamie Slabber has joined Swindon Town on loan until the end of the season.