"Energy efficent vented tumble dryer"
There is no such thing.
If it has to be vented and cheap, go with the cheapest timer based model with reverse action. Reason being that the reversing on these is verry consistent cause its timed, and frequent predictable reversing is a godsent with beddings.
Sensor drying only gets good more up the line. And durability wise, there really isn't much to go wrong.
This for example is a verry basic, well known Indesit model.
http://ao.com/product/idv75-indesit-eco-time-vented-tumble-dryer-white-15791-19.aspx
However if sensor based drying is wished upon, which I'd totally understand, Hotpoint has the widest range of sensor dry dryness degrees and a good price. Though, durability wise, today, you can't expect much from anything.
http://ao.com/product/ftvfg65bgg-hotpoint-vented-tumble-dryer-silver-48495-19.aspx
Alternativley, if the Whirlpool cooperation isn't for your likes, AO has a pretty well specked Candy dryer on offer which should have decent sensor drying as well.
http://ao.com/product/csvv9lg-candy-grando-vita-vented-tumble-dryer-white-49150-19.aspx
I'd suggest a heatpump dryer, but the cheapest start at 350£ and up, with decent units for 380£.
This Beko is A++ (means uses less then half the energy a vented machine would), and generally, these work pretty decently in terms of tangeling.