Quo Vadis, that when we saw some lovely fillets of smoked eel last weekend (at the CLA Game Fair, our annual encounter with the landed gentry) we grabbed it in order to make our own.
When we were at Quo Vadis Paul couldn't convince our sister in law to have a taste of his sandwich; she was horrified at the thought of eel. But smoked eel comes as a clean, white, boneless, vacuum-packed fillet: completely innocuous. So maybe if you are making this for someone squeamish, call it "smoked fish".
Jeremy Lee has been making his for twenty years, and it's as fine a sandwich as you will find. He has also published the recipe. But of course, I tinkered. Not in any big way, I just added some slices of cucumber. Watercress, or even better, mustard cress, would be lovely too if you can't leave well alone. Or maybe substitute wasabi mayonnaise for the horseradish. And try to cut your bread a little thinner than I did!