Obviously it's up to you what route you take but "professional advice" may well be costly and only the same as what's been said on here and what you'll be told at Sol D'Ebre.  They buy their bits and pieces, chemicals and offer the same advice as from Sospalm. http://www.sospalm.com who are apparently the leading firm of experts.  Lots of info here.
I presume you've been injecting as well as spraying.  If not, spraying alone is unlikely to be effective.
How tall is your "very high" one ?  A couple of mine are about 5 - 6 meters and I just use a back pack sprayer (mochilla) with an adjustable nozzle and adjust it to a jet rather than a spray and can easily reach the crown and all other fronds this way.  Advisable to wear a face mask when doing this.