Well it depends on how much you want it, and what your appetite for risk is.
If this has been soaking the wall for years, then the opportunity for damage is high as the water will soak into the brick, freeze, expand, blow the brick, thaw, contract etc etc. This could potentially require a rebuild of the wall if the damage is severe enough. To be fair, this is unlikely but it depends on how long it's been manifesting, and what the weather conditions have been like.
Personally, I would probably make the offer as you would, subject to survey. Tip your surveyor off that it needs to be mentioned in the survey. Once you've had the survey done, and have a worst case estimate of cost, and depending on what else is found in the survey, I would say something like "the survey has identified the issue of missing downpipe. Full rectification cost could be as much as £xxxx however to avoid protracted negotiation, I am willing to split the risk and reduce my offer by half of £xxxxx, alternatively I would continue with my offer of £yyyy if you rectify the issue to the satisfaction of my surveyor".
So in your example, let's say that the rebuild of the wall would be £30k, you could suggest a reduction of £15k thereby splitting the risk. If you don't need to do that amount of work, then you are better off by £15k, however if you do need to do all of the work, you are worse off by £15k, and you are betting on whether you have to do that amount of extreme work.
You can flex the numbers depending on how likely your surveyor thinks full rectification work being required is, and your own appetite for risk.
Does that make sense?