After my stop is hit, I want to check a condition and then place an order (which would be executed on the open on the next candle, after the stop is hit). However my understanding is that the stop order is executed after next is called, so it wouldn't execute the order on the candle immediately after the stop is hit, rather it would get executed on the 2nd candle after the stop is hit rather than the first.
Is there a way to solve this problem?

Okay fixed that issue as well using cerebro.addobserver(bt.observers.BuySell, barplot=True) .

Okay I managed to fix that by adding stdstats=False and manually adding the observers. However I noticed one of the sell triangles is behind the candle

You can see that a trade is closed in the top observer and if you look closely you can see a red triangle behind the candle. Not sure if this is a bug or something I've done wrong.

I added a second timeframe and the panel shown above used to show "Trades - Net Profit/Loss" panel with red and blue dots. Now it doesn't show the colour of the dots so I don't know if the trade is a profit or a loss. Is there some sort of way to configure the observer so it goes back to how it was by default? Why does it change so the image as shown when I add a second timeframe?

Thanks for bearing with, I added

cerebro.broker.setcommission(leverage=100)

now everything works how I want it. Thanks.

I thought you could use leverage/margin in backtrader? I can use it for shorts it seems but not longs?

Okay I will post the code here as well

import backtrader as bt
import pandas as pd
import datetime
# get data
data = pd.read_csv('https://pastebin.com/raw/8gah3XhH')
# convert timestamp to datetime object
data['timestamp'] = pd.to_datetime(data['timestamp'], utc=True)
data.set_index('timestamp', inplace=True)
class TestStrat(bt.Strategy):
    params = dict(
        printout=True
    )
    # Stuff for logging
    def log(self, txt, dt=None):
        if self.p.printout:
            dt = dt or self.data.datetime[0]
            dt = bt.num2date(dt)
            print('%s, %s' % (dt.isoformat(), txt))
…            self.log('Order Rejected')
        # Sentinel to None: new orders allowed
        self.order = None

    def next(self):
        if self.data.datetime.time() == datetime.time(8,40):
            self.buy(size=3.63)
# Create a cerebro entity
cerebro = bt.Cerebro()

# Add strategy
cerebro.addstrategy(TestStrat)

# Pass it to the backtrader datafeed and add it to the cerebro
data = bt.feeds.PandasData(dataname=data)

# Resample it to 5 min bars
cerebro.resampledata(data,
                     timeframe=bt.TimeFrame.Minutes,
                     compression=5)

cerebro.broker.setcash(10000)

cerebro.run()

What prints to the console:

2019-10-24T08:40:00, ORDER ACCEPTED/SUBMITTED
2019-10-24T08:45:00, Order Margin
2019-10-25T08:40:00, ORDER ACCEPTED/SUBMITTED
2019-10-25T08:45:00, Order Margin
2019-10-26T08:40:00, ORDER ACCEPTED/SUBMITTED
2019-10-26T08:45:00, Order Margin

A snippet of the data

2019-10-24 08:38:00+00:00,7440.5,7440.5,7434.0,7436.0
2019-10-24 08:39:00+00:00,7436.0,7441.5,7435.5,7441.0
2019-10-24 08:40:00+00:00,7441.0,7443.0,7441.0,7443.0
2019-10-24 08:41:00+00:00,7443.0,7446.5,7442.5,7444.0
2019-10-24 08:42:00+00:00,7444.0,7444.5,7444.0,7444.5
2019-10-24 08:43:00+00:00,7444.5,7444.5,7444.0,7444.5
2019-10-24 08:44:00+00:00,7444.5,7445.0,7444.0,7444.5
2019-10-24 08:45:00+00:00,7444.5,7444.5,7441.5,7442.0
2019-10-24 08:46:00+00:00,7442.0,7442.0,7441.5,7442.0
2019-10-24 08:47:00+00:00,7442.0,7446.5,7441.5,7446.0
2019-10-24 08:48:00+00:00,7446.0,7446.5,7443.5,7444.0
2019-10-24 08:49:00+00:00,7444.0,7444.0,7443.0,7443.0
2019-10-24 08:50:00+00:00,7443.0,7443.5,7443.0,7443.5

I've stripped the code and data down and stuck it in a google colab here https://colab.research.google.com/drive/14Jz3DYBjIejVIcir4BInlw1kUF2hEOJr . You can run it in your browser my pressing the play button next to the code. As you can see, same problem still occurs.

Seems to work when I do

self.buy(size=3.63*0.3)

I can't understand why I'm getting margin called? I have no other positions open, no other orders?

Okay, so I stepped through my code. The stop gets hit and the buy order does get submitted in the right place. The problem is in fact that I'm running into order.margin, which prevents the buy order being taken. I'm logging everything and I can see there is no additional orders are before this. My self.broker.get_cash() value is 9801.0 and I'm trying to submit this order

self.buy(size=3.63)

The close of the candle when this is submitted is 7443.0 and the open of the candle after is also 7443.0. Yes I am using leverage but surely this is still fine?

"Order.Margin: the order execution would imply a margin call and the previously accepted order has been taken off the system"

Why would I be getting margin called with these values?

To clarify further, my issue isn't with the execution of the stop as @ab_trader suggested, that works fine. The issue is with placing an order after the stop has been executed and having it execute in the following bar.