Unless the conduct of the solicitor that you are complaining about is conduct which is likely to bring the profession into disrepute, I’m afraid that your complaint may fall onto deaf ears. If you have found the other solicitor to be aggressive and forceful and slow replying to correspondence, they may just be doing that as a tactic.
It would be totally different if this was your solicitor that you were complaining about but the general view of the regulator, when the other side complains about a solicitor is that they are just aggrieved that the other solicitor has been more aggressive and forceful and effective than their own.
Whether the complaint would bear fruit therefore would depend on exactly why the complaint was being made. I have been involved with several complaints made against solicitors when the complainant was not their own client the client of their opponent and none of the complaints went any further than the regulator simply writing to the solicitor being complained of and also writing to the complainant in similar terms. None of the complaints went further than that initial letter.
I think you probably do have some grounds for a complaint here if they are holding funds to your order and they accepted the word of your ex rather than your own solicitor or you.
Whilst, from your version of events, they shouldn’t have done that, the complaint will be more substantial (rather than a simple slap on the wrist, “don’t do it again”) if it’s caused any problems. You do not mention if it has caused any.
Can I clarify anything else for you?