How much to invest with one platform?

Agreed, I don't buy this 100% loss, it is property after all. Worst case you would take a big hit, let's say 40%, but you don't have to earn 12% for too many years, to have a buffer for those losses.
I think SME lending is far riskier in most cases, as their assets have often vanishished by the time the liquidators turn up, then you have all those recovery experts to pay for, before you get a penny.
One loan I was in had over £2m of assets against 750k Loan, looked good until all of those assets turned out to be worthless. Others don't even have security.

One thing to consider is that no matter how robust a platform's disaster plans are, if the platform fails and the plan has to be put into action it will take some considerable time to sort everything out.  There's obviously going to be a big problem if the end result is a large loss, but even if the final result is full recovery, or a small loss, you will be without access to your funds for a lot longer than if the platform was still trading normally. 

As long as you can survive the consequences of minimal access, this may not be a significant issue, but it does provide another reason to invest in multiple platforms rather than just one.

Oh I'm quite happy to risk mine too, obviously, .. but that's different from leveraging it by borrowing and risking that. Mine I don't have to pay back anyplace. I have no problem with risky lending, I do have a problem with excessive leveraging.
ZDP ITs a few years back were another case in point. Same bad result.

There's no general numeric answer .. you have to consider all your resources, and how they are deployed. Some BHs can't be bothered with anything unless they can get £100k in . Some folks shouldn't have more than a tenner in. I'm happy with a k or two in most of them, but it is money I can afford to lose, or wait a long time to get out again, and my liabilities are few, and well covered.

I believe the biggest risk in the bridging loan & development loan market is financing/refinancing risk at the end of term. Unlike residential mortgages where there isn't expectation of any transaction that can be affected by market conditions (except where the borrower loses their job), the repayment of bridging/development loans is dependant on obtaining refinancing from a traditional lender or sale of the development - both are very sensitive and exposed to changes to market conditions. In 2008 we saw that market sentiment can change fast and quickly affect market liquidity (both in terms of credit and property transaction vols).

There is a reason the market demands 12-18%pa+ on these loans, there is quite a large systemic market risk. That said, it is difficult see more than a 50% loss in most scenarios (maybe on aborted developments if credit completely dries up and the bottom falls out of the housing market?)

My bigger concern is platform risk. Regulation by the FCA is welcome, and gives some comfort that most charlattens are kept out and platforms are managed appropriately with adequate resources and appropriate internal controls. However, fraud or some other control failure leading to a catastrophic event can never be completely discounted. Madoff was subject to regulator and auditor scutiny and he managed to pull of a $50Bn fraud and others will no doubt follow irrespective of additional regulations and controls put in place. I hope that in time that P2P platforms will be afforded protection under the FSCS.

Before investing on any platform I perform some degree of cursary DD, i.e. reviewing statutory accounts (although most are not audited), e.g. SS' unaudited accounts for the year ended Aug 15 show net assets of £270k, understanding their business model and how they make money, and how responsive they are to investors. Ultimately I only invest on any one platform the amount that i'm willing to lose and try to diversify as much as possible as some platforms will fail, frauds will occur, and people will suffer loses....never stick all your eggs in one basket.