We are very close to completing our house. Can anyone who has sold advise how the utility bills, Community fees & non-resident tax is sorted as at Completion date? TIA

You can try and agree with the new owners a direct transfer of utilities if not you should cancel all contracts on the effective completion date.
Community Fees - you should get a statement from the Community Administration at the time of sale
Non-resident tax is a retention that the buyer or their representative should make at 3% and they should declare this within 30 days of the sale. The Tax Authorities then do their official assessment and either charge you more or give some back..........
You should also provide a justification of IBI payments being up to date.

The 3% retention is in respect of any potential Capital Gains Tax liability, I believe, not the Non Resident tax.

https://www.spanishpropertyinsight.c...%20been%20paid.

It is also usual, if the seller is non resident, for the buyer's lawer to retain whatever amount is due in Plus Valia to the Ayuntamiento, which again should be paid within 30 days of the sale completion. I know the way in which Plus Valia is calculated has had to be changed recently due to the Supreme Court judgement but am not sure how this has worked in practice.

With regard to IBI, officially the owner of the property as at 1st January is liable to pay the IBI for the whole of the ensuing year. However, in practice most buyers and sellers come to an agreement that they will pay pro rata. That's what happened both when I sold my old house and bought the new property. The buyer's lawyer retained an amount to cover the period I had owned it up to the buyers taking possession, and when I bought the seller agreed to pay his share up to the date of completion.

This might help

https://expatsmagazine.org/selling-property-in-spain/

You need to be organised with everything ready for the notary process if you are uncertain employ a professional it is better than having problems at the notary and it not being completed.