What model and year is your Midget? You say it is positive earth. Are you sure? When you replaced the sender did you ensure it was the same type? I recall there are three different types of sender fitted at various times. The sender should match the guage. Also have you checked all the connections especially those between the guage and the sender. The voltage stabiliser is found on the firewall up under the dash. It is likely to be a small little metal oblong box about an 1" long and 1/2" deep. Check the connections to this.
Also worth checking the sitehttp://www.mgcars.org.uk Go to the Midget and Sprite Technical BBS and then to the Archive section. Lots of info in there.