I have a 145 with no air/con and I changed the rad pretty easy with the bumper in place. In fact bumper-off was a no-no for me since the bolts were seized... the support arms were twisting and one of the bottom bolts sheared off rather than come undone. (Job for later!)
Remove the header tank cap and take off the bottom hose to the rad' to drain the coolant. You want it to gush out, since that shifts any loose bits of scale or crud in the system.
Catch it in a basin and put it in a plastic container which you can take to your local tip. Don't flush it down the drain. It's particularly nasty stuff.
Now flush the system.... either by refilling the system with cold water and running the engine for a bit (you don't need to refit the clamp... it'll leak a bit but no bother) and then re-draining as above.... or by forcing water up the bottom hose. I use the previous method since it's less messy. Do it a couple of times to get all the loose bits and crud out. The water should come out clean and colourless when it's "done".
Now... remove all the hoses to the radiatore..
Remove the grille above the bumper. Unbolt it at the top and pull it straight out. N.b. Prise each end out carefully from the outside in first. The ends are held in place by push fit spring-clamps but they'll be stiff. If you try to pull the grille out holding it from one end, it'll twist and crack. You have to pull it off "straight"..
Unbolt the top valance panel (matt black on mine). Leave the bonnet release cable on there.
Remove the radiator fan and the electical connector to the water temperature sensor in the rad. If I remember, I dropped the fan out from below.. but it could have just lifted out with a bit of juggling.
Pull the rad out of its mounts. It has 2 plastic lugs which fit into rubber bushes in the radiatore support bracket. You might want to buy two new rubber bushes since it's a once in an ownership job.
Use the opportunity to rust-treat your radiator support bracket. Mine was turning to rust. De-rust it, treat it, stick zinc primer on it, paint it... waxoil it, treat it again.... It's hard for it to survive down there.
Also raise the car up and get her head in there from below. You may seem some other skankiness that you may want to clean up.
New rad drops in. Use a spot of Fairy Liquid in the rubber bushes if you use new ones.. It helps the radiator lugs to slide in there. Not vaseline since it will rot your new bushes.
Put everything back on in the reverse order. Add new coolant as slowly as possible, so that air has a chance to escape. Use the pink stuff diluted to 50%. I buy the concentrate and dilute it myself wuith distilled water since it's better value... but the pre-mixed is the same stuff.
Start the engine and let it idle for about minute, with the heater on max (it lets coolant through the heater matrix to bleed the air out). You don't need the fan on.. since that just cools it down. Dunno why the books say the fan should be on actually.. The idea is to let the coolant circulate and it can do a lap or two in a minute. Don't let it idle for long enough for it to heat up.
Switch off, remove the header cap (if it's vaguley warm, be careful.. but I did say just let it idle for a minute..
) and top the level up (it will have dropped).
Refit the cap, check your hose clamps (dry off any drips or wet marks so you can see that it's not leaking) and go for a little drive. When the beast is cool again.. have a peek underneath in case the clamps are leaking etc. but it should all be dry and splendid.
Have a look again the next morning.. and add a bit more coolant. It always drops overnight... but then that really should be it.
I would invest in new
radiator hose clamps. They's about £1 ..and worth the aggro since they're going to be a bit corroded and shonky. My top-tip for hose clamps is when you get to the point when they're almost fully tight, back it off a turn or two and stuff some gasket sealant into the screw, before tightening them.
That acts like a plug... keeps crap out ...so that it comes undone the next time someone wants to unscrew it .. and it locks the thread, so that the clamp doesn't loosen off or vibrate undone.