I have just passed my grade 5 tuba after a term of lessons. I would be grateful for any advice on technical studies etc that I should be looking at as I am hoping to do grade 8 in the near future.

Also what pieces do you recommend.

I have played the Alan Ridout saxophone concertino - is the tuba one any good?

Well i'd go for any studies by Derek Bourgeois (Per Tuba ad Astra, Fantasy Pieces) as they're very melodic and also can provide quite a challenge to your technique.
Im not too sure what gde8 pieces are on the ABRSM as i teach Guild Hall.
Sharpey

The Derek Bourgeois Fantasy Peices are the National Youth Orchestra auditoin pieces, written for every member of the orchestra. Theyre reasonably hard, but his tuba concerto is harder - very good - got mine for just £5.95 from my local music shop! i recently got a lot of good gr 7/8 tuba repertoire as reccomended by a conductor friend. They include:

Capuzzi's Andante and Rondo (for double Bass)
Six Studies in English Folksong with the Tuba alternative part
Vaughan William's tuba concerto
the brilliant Senaille - Introduction and Allegro Spiritoso

The Senaille is particularly good as is the Capuzzi.

Hope this was useful!!

Ps A good book for studies is either Tuba Eurythmics by Alwyn Green (with some fantastic unaccompanied Bach near the back)
and
the Arban cornet method, sometimes a little high, but great for technique