A more fundamental question is whether you should continue to hold the buy to let.
If you have a very good tenant paying a good rent and it's no hassle, by all means, continue to hold onto it.
But if the tenant leaves, then I think you should sell it. With your level of wealth, you don't need the potential hassle of being a landlord.
If it's potentially useful for the kids in college, then maybe hang onto it.
But don't hang onto it because you would get less than you paid for it if you sold it. You can use the CGT losses against future gains.
Ah, that is another, albeit minor, reason for investing rather than paying off your mortgage. With an unrealised €70k CGT loss on the property, you might be able to set it off against gains on any equity investments.