Heat the oil in a deep-fat fryer to 180°C/350°F, until a small piece of white bread turns golden brown in about 30 seconds; or use a deep-sided pan and ensure that it is only half way full.
To make the beer batter, sift the flour and cornflour into a bowl. Make a well in the centre and add the lager and mustard.
Whisk the liquid ingredients together and then gradually whisk into the flour mixture until you have achieved a smooth batter. Season to taste.
Dust the cocktail sausages in a light coating of flour and then dredge in the batter, shaking off any excess. Deep-fry for about 5 minutes or until cooked through and golden brown. You may have to do this in batches depending on the size of your fryer.
Meanwhile, make the mustard mayonnaise, place the mayonnaise in a bowl with the mustard and lime juice. Season to taste and mix until well combined.
To serve, drain the beer battered cocktail sausages well on kitchen paper and stick on to forks or cocktail sticks. Arrange on serving plates with small bowls of the mustard mayonnaise on the side for dipping.