Daniel Stanton's response

This might be a bit late in the day but hopefully this answer helps others in a similar position.
You mention that there was no mention in the contract of the buildings insurance and ground rent. It might be of some comfort to know that most residential sale contracts for property in England and Wales incorporate a set of 'Standard Conditions', which deal with a lot of the routine issues on a house or flat sale. There are different formats and versions of Standard Conditions, but most solicitors I come across have settled on the 4th Edition of the Standard Conditions of Sale. There is no need to attach a copy of these Standard Conditions to the contract itself, although some solicitors do it out of courtesy.
The usual position under the Standard Conditions is that a tenant selling a flat is required to make sure all the ground rent and service charges are paid up to date by the date of completion. This means paying in advance for any periods after completion if the due date falls beforehand.
Further obligations in the Standard Conditions require the buyer to pay apportionments for periods after completion and add these to the purchase price. The buyer's solicitor would usually deal with these, but may rely on a 'completion statement' prepared by your own solicitor setting out the amount due.
It is surprising that your solicitor has not put your mind at rest over an issue which is clearly important to you. Perhaps you were using a call-centre style operation, where technical questions about the contract are sometimes ignored through lack of qualified staff, or at the other end of the scale, perhaps you used an old-fashioned solicitor who may have assumed that you were trusting him or her to deal with all of these things and s long as they dealt with it all, you would be content in the end. As you might expect in any walk of life, there are different solicitors who might have different approaches to how they deal with such things. It's easy for me to say that of course I would have responded to you, but on a day where there are a dozen completions scheduled for the following day, perhaps there was simply not time (although a quick message along the lines of 'all taken care of' wouldn't have taken too long to get out to you).
I hope it worked out for you in the end and that this reply is of some comfort. You will appreciate I don't have full details of your sale and accordingly it's impossible for me to give any more than a general answer, the accuracy of which will depend on the facts of your particular transaction. If your solicitor was unable to deal with your request, you will be able to seek a second opinion, but unfortunately this will nearly always mean further legal fees. You may be best making a complaint to the firm first of all to see what they have to say.