The egg sac will be useless. First of all, the bananas will have been kept at very low temps; killing the eggs. Phoneutria spp. need high humidity, and the long ride to the UK would have had lots of flutuation in humidity as well as temp (esp if in a cooled room or a fridge). The egg sac from this species (like other spids) needs to be able to breathe, and to do this the female usually 'rolls' the egg sac. In this species the female is known to keep a very close eye on the sac, and will regually nudge, or even carry the egg sac about so the eggs are able to breathe and are safe. If the sac was left on the bananas without having been turned or nudged, and there is a web covering it, the chances are they are almost certain to have died long ago. The webs they use to cover the egg sac is very strong and lets in very little air, and the female will usually keep cutting holes in it and rewebbing it back up.
But, just to be sure, I would say to put it in the freezer. This species of spider is quite possibly the most dangerous spider on earth, and they are VERY aggressive. I have personally kept a few of these, and they are one of the only spiders I've kept that will stand it's ground. It's like they know they can kill, and they will if they feel threatened. So, if you have a sac of these hatch, you will deffo be in for a real supprise, and tbh; even though I love my dwa inverts, I wouldn't even bother incubating them.