Put in fifth from under the bonnet using the main shaft off the gearbox. Start the car (with the cluth depressed of course) and let the clutch in slowly to see if there is any 'drive'. Clearly you're not going to get rolling in fifth but you'll soon know if you stall the engine that the gear is there and its probably a linkage problem. Not getting fifth in my experiences with mk2's is the plastic bush next to the exhaust that the gear change shaft runs through gets totally shot, often but not always accompanied by difficulty in getting reverse or first. Usually worse when everything is warm as the plastic gets softened from the exhaust heat. If the gear is there then you'll need to sort out the linkage, and if you haven't had the car long get the gearbox oil level checked out quickly. Two out of the four I've had only had half a litre in when I got them, they should have 2, and I believe fifth is the first to go if they run dry.
