Sorry -- haven't been home long and this, of course, didn't show up on my email. Benedryl -- 1-2 mg/lb body weight given two to four times a day.
So if she weighs 50 pounds you'd give 2 of the over-the-counter capsules (make sure it is JUST diphenhydramine -- not 'severe cold formula' -- nothing with decongestant or acetaminaphen in it - that's critical! Tylenol/acetaminaphen is *poison* to dogs and can kill in small doses)
Here in the States, Benedryl is 25 mg in the OTC capsules/caplets and 12.5 per teas. in the children's "elixer" formula.
For a spider bite I'd probably do the 2 mg/lb and do it at least 2 times a day. It will help her body cope with it and it will help her not scratch it into an acute problem. Now it *will* make her sleepy -- but that's ok. That's part of keeping her from bothering it.
Spider bites can HURT ... a lot. I can cause pain to radiate up and away from it. So she could be a little bit out of sorts or acting 'off'.