Here's what I went for... After quite a lot of research around the web.
For those who don't know, the Fender Blues Junior (as with other tube amps) comes with 2 different types of tubes. Firstly the output tubes (often coming in matched pairs, important to check this out for your specific amp) and secondly the pre-amp tubes which generally come in 3-5 stages. The Blues Jr has 3 stages - with the order / type of tubes making a difference here - V1 is the pre-amp gain stage, V2 the reverb and V3 the phase inverter tube.
For the EL84 (output) tubes, I purchased - EL84/JJ-TESLA (JJ-Tesla) from Watford Valves. This has given the amp a new lease of life and a cleaner, punchier sound. The output tubes are likely to 'go' before the other pre-amp tubes.
For the pre-amp (12AX7) tubes I purchased - ECC83/12AX7A/TUNG-SOL/RUSSIA (TUNG-SOL RUSSIA) and 2 lots of ECC83/ECC83S/JJ-TESLA (JJ-Tesla). I put the Tung-Sol tube in the V1 gain position as recommended by people and have been pretty happy with the sound!
Overall, the tube updates cost around £40 from Watford Valves who are always very happy to give you advice (highly recommend these guys). Phone them or wait a few days for an email reply, which they will unlike so many other companies!
Change tubes when you notice a change in colour on the tubes, notice a power loss, hear a ear-piercing shreek, amp becomes more noisy or hissy (don't forget this can be due to pedals, patch leads, power cables, tube lighting in the area, or another amp fault). Or when you feel a change is needed. People commonly swap out the output (EL84 type) tubes around once a year or every couple years.
The sound difference isn't absolutely mind blowing, but it is very clear, more distinctive, refined and has more clarity/finesse. Well worth doing!