The analyses of a text #1

The narrator recalls his adolescence.
He, his great aunt and friends came to Ham's office to meet Mr. Micawber who is the clerk for local lawyer Ham.
Ham is the partner of Mr. Wickfield whose only daughter is narrator's old friend.

Chapter 52 I ASSIST AT AN EXPLOSION
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'You are not busy, Mr. Heep?' said Traddles, whose eye the cunning red eye accidentally caught, as it at once scrutinized and evaded us.
'No, Mr. Traddles,' replied Uriah, resuming his official seat, and squeezing his bony hands, laid palm to palm between his bony knees. 'Not so much so as I could wish. But lawyers, sharks, and leeches, are not easily satisfied, you know! Not but what myself and Micawber have our hands pretty full, in general, on account of Mr. Wickfield's being hardly fit for any occupation, sir. But it's a pleasure as well as a duty, I am sure, to work for him. You've not been intimate with Mr. Wickfield, I think, Mr. Traddles? I believe I've only had the honour of seeing you once myself?'
'No, I have not been intimate with Mr. Wickfield,' returned Traddles; 'or I might perhaps have waited on you long ago, Mr. Heep.'
There was something in the tone of this reply, which made Uriah look at the speaker again, with a very sinister and suspicious expression. But, seeing only Traddles, with his good-natured face, simple manner, and hair on end, he dismissed it as he replied, with a jerk of his whole body, but especially his throat:
[David Copperfield by Charles Dickens]
1. I'd like to know why it is "not but what," not "not but that."
2. And I'd like to know if "it" refers to "a very sinister and suspicious expression."
Thank you in advance for your help.