Mix the crab and shrimp together in a bowl with the red pepper, celery and spring onions.
Add the avocado and stir to mix through.
Stir in the lemon juice.
Sprinkle with the celery salt and black pepper.
Gradually add the mayonnaise, if using, a bit at a time, until the mixture just clings together.
Spoon into pretty bowls and serve. I like to make sure there is a shrimp or two on the very top of the salad, it makes it look pretty. Or use as a sandwich filling or to top crackers or melba toast as hors d'oeuvres.
Garnish with paprika or chopped, fresh parsley.
(To prepare ahead of time, prepare to the end of step 4, cover and keep in the fridge until ready to serve. Portion out and garnish just before serving. Best served on the day it is made.)