Leaky thermostat - which one to replace?

[akiller]

Hoping someone can help. Apologies for the noob question but I'd sooner make sure I get it right:

My car (phase 1 XUDTD) has been leaking coolant for a while and it appears to be from around the thermostat housing. The existing seal on the thermostat has a slight nick in it and I temporarily bodged it with gasket seal but there's still a little bit getting out.

I figure I may as well replace the whole thermostat at the same time with a cheap and cheerful one.

ECP seem to have two which are ever so slightly different. Which is the one I should get?

• http://www.eurocarparts.com/ecp/c/Peugeo...f81&000325
  
• http://www.eurocarparts.com/ecp/c/Peugeo...8b2&000325
  

Their description of "with seal, without gasket / seal" is somewhat confusing :p. Assuming it doesn't come with a seal, is http://www.eurocarparts.com/ecp/c/Peugeo...afc&000574 likely to fit both?

I assume it's just a case of undoing thermostat housing and taking the old thermostat out, putting the new seal on the new thermostat and bolt it all back up again (+ refill/bleed lost coolant)?

I'm reluctant to undo the thermostat housing to measure the current one as I'd rather not disturb the existing seal and what's left of it.

Cheers.