Nokia :: N97 Battery Won't Charge BL - 6f / BP - 4L

After finally getting rid of my 5800 doorstop, I'm now the (proud?) owner of an N97. I've encountered a fair few of the software/ c: problems and using many of the usefull tips to help with those, BUT - after 2 weeks my supplied battery (BP-4L) now won't charge more than half full and when used it dies after minutes when using the internet or an application. Thats just not right, it got an initial 24 charge and bi nightly, then nightly recharges in the first week, and its having to be left on charge constantly now.

So I rang my SP Vodafone as it was only 2 weeks old, and the chap said the 4L was the wrong battery? he said it should be a 5f but as he had none he would send me a BL-6F which was compatible and would last longer. Can't believe this will be correct, surely its a smaller battery, its also 1200mah as opposed to 1500mah so will drain quicker. Any Techy chaps know whether this'll be acceptable. Drawing up the strength for another bash at the SP, but its like speaking dutch to them.