I searched on FindMyPast.co.uk in their "Devon baptisms" collection for William Wills baptized in 1804 plus or minus two years. There were 18 of them. It was not an uncommon name. However, there was exactly one baptized in Marldon in 1804, and I have no doubt that is the right one. Of course, it would cost me money to get the details, and I am not going to do that. (I don't even seem to be able to buy "pay as you go" credits on FindMyPast anymore.) Given that at the 1871 census Dr Wills said that he was born in Marldon, and 1804 fits with his age, I think it must be right.