Anyone have problems with consistency and sort of 'running with it' when practising? 

I'll normally pick up the guitar with no expectations of sounding good after ive given up in disgust after a previous occasion, then I play my favourite riff, see ive got a bit better since last time, but as soon as i think im doing ok and should practice more to get to the 'next level' its all down hill, and the day after when i pick it up i just cant do it anymore.

Anyone got any good advice on this matter?
There's a big difference between playing (for fun, relaxation, gig ...) versus practice.  Practice is all about improvement.  Trying to turn your worst capability into ytour best, and acknowledging that this can be hard. There's no point in repeating stuff you can already do.  Practice is also about mental attitutude ... which is probably half rthe battle.  Pick up a guitar with the attitude that you're going to improve and enjoy yourself. and with sensible targets, and you will improve.  If you think you're going to suck, tthat's self fulfilling.  

Sounds like you could do with a decent practice routine ... but most importantly, you need to work on tunes, others or yours.  The more of that, the more you expand your awareness.

Try some different stuff.  That can stimulate also.
Whenever I start to get dissatisfied with my playing, I practise rhythm and timing against a metronome, and it's like an instant improvement every time.
Don't let your expectations exceed your effort. If you haven't put the hours into making something sound good... it's not going to. You just have to put the time into practicing the specific things you want to sound good, and over the course of hours/days/weeks, they will sound better.
I have a bad habit of practicing too much on one weak area of my playing. I'll be learning a song that has a chord voicing/transition I'm not used to for exmaple and I'll just practice that one chord change for like an hour. And obviously, in that hour I barely improve. It's much better, and less stressful, to set the timer for 10min, practice it and move on to the next thing (without getting caught up in how good or bad the last 10min went). Doing this every day you will definitely see steady improvement over the week(s) ahead.