I presume that the rectory no longer belongs to the church authorities. If this is the case, you should endeavour to establish whether or not the wall belongs to a third party. If the church or the rectory is listed, check the listings. Check the title deeds of the rectory. If either end of the wall stretches beyond the limit of the grounds of the rectory, it is likely to be part of the church property. If all else fails, knock on the door of the rectory and ask the owner which of their boundaries they maintain (although they might seize the opportunity to give you a wall!)
If the wall belongs to the church, it is now the council's responsibility to maintain it, unless it was listed as an exception at the time the Section 215 agreement was negotiated. If it was an exception, and it remains the responsibility of the PCC, in the view of the NALC legal bods, the council may not lawfully contribute to the costs, as a specific prohibition exists in the 1894 Act. Having said this, the same rules apply to grass cutting in open churchyards, which hundreds of parish councils undertake from the precept, so a proportionate and necessary spend is unlikely to result in serious consequences.