The 410 has a replacable battery so this shouldn't be an issue for you with that unit.
It seems to always be reduced on Amazon. I picked it up for £147 with the HRM between two days when it was £159.99 so it may be worth watching it for a day or two.
You could get the 410 and in the mean time give replacing the battery a go? If it works return the 410, if it doesn't then it doesn't matter as it was unusable anyway.
Edited: 01/11/2012 at 14:07