Those facts scream "fiduciary relationship" to me. An estoppel is definitely arguable, but I think you might have difficulty showing reliance. In Cobbe, the party seeking to rely on an estoppel had spent money obtaining planning permission - I can't see any obvious reliance on these facts.
The big advantage of having a fiduciary relationship rather than a remedy in contract/tort is that the remedies are much more powerful. In contract and tort you only get compensation for loss. If you show a fiduciary relationship, an account of profits is available, and some of those profits may be held on constructive trust, see Boardman v Phipps. In addition, opportunities are property, see Regal (Hastings) v Gulliver and CMS Dolphin v Simonet. This opens the way to a constructive trust. There is extensive, controversial and messy case law on this point, mostly in relation to companies, but the principles are the same in partnerships.