For what it's worth, iZettle got back to me with a statement after I covered this on The Next Web:
"Being a startup ourselves we always encourage entrepreneurship, including innovation in the payments space. In this case, unfortunately, their very similar company name – in combination with also being in the same industry- risked confusing both our and their users. We believe that changing their name will benefit the both of us and we wish them all the best with Droplet."
I prefer the new name in any event but that doesn't take into account the goodwill you have built up with the old name. The problem is the lack of certainty over the outcome of litigation. Clearly iZettle have at least an arguable case that there is a risk of consumer confusion but obviously it requires a court's determination to provide certainty.
The problem is that the larger entity is more likely to have the resources to engage in litigation which in itself means smaller entities are less likely to fight back.
Trade mark litigation is often costly due to the need for survey evidence to show confusion amongst consumers. One benefit of going via OHIM (rather than the courts) is that costs are limited in this regard and you can get an early determination from OHIM as to whether there is a likelihood of confusion between the marks.
For example, if you applied for the 'Settle' mark at OHIM, and iZettle challenged, you would have limited costs exposure to the other side if you lost (although on the flipside, limited recovery if you won). Somewhat besides the point in the current case but I thought it an interesting point to include in my comment.
At least the existing trade marks incorporating 'Droplet' are in much more dissimilar areas so you are less likely to face issues with the incumbents.
You aren't the person courts worry about when talking about trademarks...
I don't know the history of iZettle or Settle, but it seems like iZettle is much further along. I don't know about UK trademarks, but iZettle applied for their US one in 2011. I didn't find a "Settle" mark at all in the US. Given all of that, and that they are both in the "payment processor" category, I could easily see a court siding with iZettle on this one.