And the grubs had orange heads, Balcony? VIne weavil grubs - dump the compost and grubs in the waste bin.

Vine weevils - kill them ! I squash them lol. You can get a biological control for them. You mix it with water and pour it on. Also look out for the adults and kill them. The more adults you kill the less grubs you'll get.

And the adults look like this -
http://www.forestryimages.org/browse/detail.cfm?imgnum=2130078

"And the grubs had orange heads, Balcony?"

No, they didn't, Bulbaholic. They were tiny, no more than a few mm / 1/4 in. & completely white. No markings/colouring or anything else to distinguish them. They were curled up in semicircles.

Thanks Hywel. When you said "look out for the adults" I thought, what do they look like? Reading the next post I saw my question answered!

Thank you Beattie for the link to the picture of these pests. I haven't seen anything like these on my balcony.

Pictures of both weevils and grubs can be found here:
http://www.dgsgardening.btinternet.co.uk/vineweevil.htm

Root mealybugs? These are white, 2mm long, and often you find a whitish powder in amongst the roots as well. Other option is root aphids, but these really are tiny, not exactly white, and globular in shape. If its root mealybugs, dispose of the compost, sterilise the container or wash well with soapy water, discard the plants. This pest favours several plants in pots, fuchsia being one of them, and they're more likely to occur in dry soil conditions.

I'm sorry but from what I remember of them they neither had orange heads nor was there any white powder amongst the roots.

Thank you for your efforts & I wish I'd taken a photo of them as I thought of doing at the time. Just that my hands were covered in soil & I didn't want any to get into the phone, it's one of those that slides up to open & I almost always open it to take photos. If I leave it closed I often find my finger in the picture!!!