I would sit back and wait for their response which should be within 40 working days. Hope you sent your SARN letters by recorded delivery.(posted 7626 days ago)I don't think there is any point in writing to them requesting documents as the idea of the SARN letter is to make them supply everything. If they do not reply to the SARN letter with all the relevant info, it is unlikely that they would do so if you wrote to them now.
Meantime if you feel your house was undersold use the breathing space you have to consult Land Registry for comparisons of house sales which might throw up supportive evidence if you intend to claim they undersold your home.
Hope this helps.
Moira