I deal with many boutique investment banks, M&A advisers and corporate finance houses - people who sell businesses. Their fee model is very much a small retainer of about £30K to £50K at the start followed by a success fee when the business is actually sold.
The success fee is quite large. Grant Thornton need to see a clear route to £300K before they'll take a business on. Zaoui, Houlihan Lokey, Lincoln International etc need to see £5m! The threshold varies but most need to see at least £200K if they are to not lose money on the deal.
Think getting £3,500 at the end of the job is difficult?! Try asking for £350,000.
The perennial problem with these intermediaries is that when they've sold a client's business and the client needs to cough up £300K suddenly they'll find all kinds of objections and complaints and feet shuffling and prevarication.
Even the most agreeable, friendly and reasonable clients get to that point and suddenly want to renegotiate fees or ask for a discount on some pretext or the other.
I have advised these investment banks and other intermediaries on how to minimise this problem. I'll summarise my advice here:
1. The engagement letter may have made clear that you'll get 2.5% of the headline price (including all deferred payment, earn outs blah blah). But people don't read contracts! After they've signed it, make this point in clear English in an email dedicated to spelling out to them that they'll have to pay you 2.5% (or whatever) of the price ON THE DAY OF THE TRANSACTION (even if they are only going to get paid later). Get them to confirm by reply.
2. Manage expectations through the transaction. Remind them along the way that you're entitled to £300K+ at the end. If they ever praise you for something, bring up again that you're worth the £300K you're going to be charging them. Keep mentioning the figure.
3. Consider modifying your fee structure to include milestone payments at the research stage, CIM, Heads, VDR setup etc. Installments reduce the pain.
4. Right at the start take an irrevocable mandate from the client instructing their solicitor to pay you £x out of the proceeds of the sale on the day of completion.
etc etc. (If you want points 5 - 10, I'll have to invoice you!

but hopefully the above will give you some tips)