I Have done a few welding bumpers back together now and you can use a heat gun and the right nozzels,
you can get the right sort of heat gun from:-
Pipeline and Engineering Products
Unit 1E Cricket Steet Business Park
Off prescott Street
Wigan
Lancs
Tel - 01942 825 155
Fax - 01942 821 534
Ask for Richard in Sales
I would recomend a Forsthoff Quick L Heat gun(240v) 136 + VAT
( my pound sign has stopped working on my keyboard )
you will need a 3mm speed weld nozzle 26 + VAT
it would also be handy to have a tack weld nozzle 3mm 22 + VAT
you can get polyprop weld rod from-: Buxo-Plas Tel - 01433 620 175 @ 3.80 + VAT per kg ( it comes in 2 Kg rolls and in various colours.
for ABS you can get it from Tempatron telephone - 01189 314 062 and it comes in 1 metre rods and you have to buy 1kg ( 3mm )
and its a stonkin 47.04 + VAT per kg...
to tell what material your bumper is made off it will be marked with a code. ABS etc.. if it is polypropylene it will be marked PP.
hope this is a help to you......
:D