ok so i have a second bike i want to make a single speed to give it a go.its 24 speed and i dont really want to spend any money.thats the criterior! happy to keep the front dereuilier for now and even maybe the whole cassette so i can play with ratio's etc, its just the tensioner that i need to sort.can a rear derailer be used some how? i think i have seen it in use but how is it done or am i talking total crap and i just need a tensioning doofer thingy me bob.
links appretiated where ness.

Just stop changing gear!

If you're happy to keep the front dérailleur, cassette and even use the rear dérailleur then the cheapest simplest way is just not to change gear!
Or adjust the screws on the dérailleurs so you cant change gear.

Though you will miss out on many of the benefits of singlespeeding such as less to break and lower weight, it would maybe give you a little insight?

Spacer kit + tensioner + 4*SS chainring bolts. £35

If you want it properly smooth: BMX chain and a non-ramped front ring. £22

chears daffy.can someone enlighten me as to tensioners.what would be good and i guess it fits to the hanger yes?

http://www.charliethebikemonger.com/userimages/procart2.htm

ask Charlie – super helpful

Surly and On-one do reasonable tensioners. For the most part they attach to the mech hanger and push up or down. Up is apparently better…

> ask Charlie – super helpful
+1

Chainring http://www.velosolo.co.uk/shopcrank.html £17
tensioner, spacers, sprocket EBAY LINKY £20
BMX Chain from Halfords £7

if i have semi-horizontal drop outs, as is common on really cheap frames (as this is) would this allow enough adjustment to not need a tensioner?

just to repeat this is an experiment not a "proper" build.

Surely if you want super cheap find the gear ratio you would want and then try riding without changing gear. If you're willing to spend a little get a tensioner & converter (from on-one for £20 for both) and ditch all your gears apart form still having 3 rings up front.

if i have semi-horizontal drop outs, as is common on really cheap frames (as this is) would this allow enough adjustment to not need a tensioner?

Maybe but it depends how much room you have to play with (up and back I assume). A half-link or a half-link chain might make it more do-able/easier without a tensioner.

I converted for about £2.

Old cassette spacers to pad out the freewheel with it's one cog, taken from the old cassette.
Shorten the chain a bit (trial and error)
Old knackered rear mech as tensioner, a good one would work perfectly well too if yours is fine. Set the limit screws in the right place.
Remove front mech, shifters and cables.
Spent the £2 on some shorter chainring bolts. Use middle ring in middle ring position, adjust chain line by moving the spacers around.