Hi Rory welcome to the forums. 1st off where are you and the bike? The US Europe? Does this look like the original color? How about a little more information? What components are/were on it? What size is the seatpost and any idea on the bottom bracket threading? Is there anything stamped on the dropouts?
It looks like it has an OK stem but the crank looks like a lowend model with the chainwheels swedged (pressed) onto the arm.
Where did you check for a serial? Under the BB shell, seat lug area, along the bottom area of the seattube, the dropouts, headlugs?
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk