I have just got a second hand bike and the pedals are seized in the crank arms. I want to change them for SPDs but cant seem to budge them. I've so far bent a cheapy pedal spanner trying to get them out.

anyone got any tips on getting them out? Should i clamp the pedal in a vice and give the crank arm a good thwack with my hammer or is there a more subtle way of getting the buggers out?

I've use the 'pedal in a vice and a big hammer' method. It was the only way on my old commuter cranks. Soak with plusgas or wd40 first.

Heat.

how do i apply heat to the cranks then TJ?

oh and probably teaching granny to suck eggs, but make sure you are undoing them the right way. I only mention this by way of confessing a similar half-hour altercation with an old bmx several years ago.

If the bike is built up, I like pulsgas and 'gentle but long' leverage eg big vice and big spanner or finding a way to prop yourself against a wall and using your body weight on the opposing pedal: i seem to have had better results with this way with old seized in bolts than anything 'percussive' as it seems to round off the flats a lot faster. Certainly worked on my car a few times as well as a few pedals, bottom brackets and cranks.

A proper big size/wide jaw pedal spanner isn't that expensive from halfords (its a rebranded cyclo one), you can use the 2 different angles for 'optimum' leverage and will last forever.

Do they have a slot for a 8mm hex key or just for spanner?

As above for WD40 overnight then put a long handle hex key in the slot with the handle facing the back of the bike, parallel to the ground.
Sit on the bike, while leaning against a wall, fence or get someone to hold you up.
Apply both brakes and slowly stand on the key handle, should come right off.

…..but be careful not to maim yourself on the chainrings! This method works but dont go crazy and stamp on the thing…..

Heat – blowtorch. CHeap from Lidl / Aldi

proper pedal spanner! A good long-lever spanner like a Park or Cyclo tool. the cheap ones will bend first as you've seen.

Two spanner method, or a big long pole on the end of a spanner will do the trick

Proper pedal spanner, some assistance and standing on the spanner seems to work….