CFMU Handbook
5.1.4. EOBT Requirements
It is a requirement for both ATC and ATFCM, that the EOBT of a flight shall be an accurate EOBT. This applies to all flights, whether subject to a flow management regulation or not.
Any change to the EOBT of more than 15 minutes for any IFR flight within the FPM distribution area shall be communicated to the IFPS.
Therefore I read that as no requirement to file a DLA or CHG message with IFPS (and thus CFMU) if you are +/-15 minutes from EOBT. Outwith the 15 minutes, or if you have a slot, then you do have to make the changes known as per the CFMU Handbook. However, it does not directly mention whether the CTOT has a similar tolerance.
4.4.3. Slot Revisions
Revisions to CTOTs should, where possible, be coordinated between the AO and the CFMU using the ATFM message exchange procedures. However, it may be the case that last minute revisions to CTOTs and slot extensions when the pilot is in direct communication with ATC, are more easily or efficiently coordinated with the FMP/CFMU by ATC.
Now, this is something which can be used and is a CFMU 'rule', so it will keep the Spanish happy. The aircraft is allowed to taxy 15 up to minutes early (with no slot) as per 5.1.4 above. ATC should therefore be able to work out how early the aircraft might be able to depart and can 'easily or efficiently' co-ordinate this with FMP/CFMU. Simple. You make a call and then the Flow people give you the 'permission' you need to let the flight go early. Everyone is in the loop, especially in the wider ATCFM sphere, and the pilot is happy. It's called providing a service. AENA, the Government, and anyone else with an axe to grind can't do anything against the ATC controller. He has complied with the European wide procedures. If Spain (AENA or the Government) don't want to abide by the European procedures laid down, then don't bother to be part of the CFMU system. Sort it out yourselves. That would of course be a very backward step, but if you're in the game, you should play by the rules which are available to you in order to provide the best and most efficient service to your CUSTOMERS.