Sanitize prices: drop out-of-band rows (50-500p) that poisoned the cheapest reference and deltas

app/main.py

@@ -215,22 +215,33 @@ def _fuel_price(station: dict, fuel: str) -> Optional[float]:
     """Extract a price for the fuel grade from a station dict.
 
     The Fuel Finder payload nests prices under keys like 'prices' -> 'E10'.
-    Tolerates both flat and nested shapes.
+    Tolerates both flat and nested shapes. Returns None for prices outside a
+    sane band (50-500 pence/litre): the dataset contains junk rows (e.g.
+    1.299p from a pounds-denominated column, or 1589p) that would otherwise
+    poison the nationwide "cheapest" reference and every delta.
     """
+    raw = None
     prices = station.get("prices") or station.get("price") or {}
     if isinstance(prices, dict):
         for key in (fuel.upper(), fuel.upper() + "_PRICE", "price_" + fuel.upper()):
             if key in prices:
                 val = prices[key]
                 if isinstance(val, (int, float)):
-                    return float(val)
+                    raw = float(val)
+                    break
     # flat fallback
-    for key in (fuel.upper(), "PRICE_" + fuel.upper()):
-        if key in station:
-            val = station[key]
-            if isinstance(val, (int, float)):
-                return float(val)
-    return None
+    if raw is None:
+        for key in (fuel.upper(), "PRICE_" + fuel.upper()):
+            if key in station:
+                val = station[key]
+                if isinstance(val, (int, float)):
+                    raw = float(val)
+                    break
+    if raw is None:
+        return None
+    if raw < 50 or raw > 500:
+        return None  # junk row — treat as not selling this grade
+    return raw
 
 
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