HJ Grundy in Bolton

got a very bad experience from HJ Grundy fish trader in Bolton Market, bought a box of prawn costs £18, a mackerel £3.10 and 405g of small silver colour fish 3.98 per kilo. Total charge was £27.55, I asked her how much for that small silver colour fish she said 4 pounds something, it meas it is over a kilo, but I don't feel it's over a kilo, but I don't have scale and calculator to calculate so I think maybe I added it wrong so I paid and leave. After got home I scaled it which is only 405g and would only cost £1.63... the mackerel is weighted 489g, £3.10 is normal. but how could all cost me £27.55? that lady over charged me nearly £5 more! how silly I am, I will not be that polite anymore, if any doubt, ask to redo everything otherwise, buy it from someone else.